fix(linux/build): pin user_name=user, mkdir derivative-binary (M1.1)

Run #4259 (the systemd-in-container debut) cleared every prior failure
class, ran for 15 minutes, then died inside 1100_sanity-tests'
aptgetopt_conf_add at:

    tee: /home/root/derivative-binary/30_derivative-maker.conf:
    No such file or directory
    last_failed_bash_command: tee --append -- "$dist_aptgetopt_file" > /dev/null

Two compounding bugs:

1. **user_name resolves to "root" via $SUDO_USER**

   derivative-maker/help-steps/variables (lines 80-93) computes
   user_name with these fallbacks, in order:

       [ -n "$user_name" ]   || user_name="$SUDO_USER"
       [ -n "$user_name" ]   || user_name="$(logname 2>/dev/null)"
       if [ -z "$user_name" ] && [ "$(id -u)" != "0" ]; then
          user_name="$(whoami)"
          [ -n "$user_name" ] || user_name="$USER"
       fi

   build.sh enters the container as root (systemd's
   docker-entrypoint.service runs as root), then sudoes to user via
   `sudo --preserve-env -u user --`. sudo always sets SUDO_USER to the
   *calling* user (= root), regardless of --preserve-env. So
   variables.sh hits the first fallback and computes user_name="root",
   then HOMEVAR=/home/root, then binary_build_folder_dist=
   /home/root/derivative-binary — a directory that does not exist
   because root's home is /root (not /home/root).

   Fix: build-inner.sh now exports user_name=user before sourcing the
   config, satisfying the first-priority check in variables.sh and
   short-circuiting the SUDO_USER fallback. The comment in the script
   notes the failure mode for the next reader.

2. **Missing mkdir of derivative-binary**

   Upstream's derivative-maker-docker-start does:
       mkdir --parents -- "${HOME}/derivative-binary"
   before invoking derivative-maker. Our build-inner.sh skipped that
   because previous iterations didn't reach the point where it
   mattered. Now that we do, we replicate it.

3. **Output collection path correction**

   derivative-maker writes its ISO/manifest output into
   ${HOME}/derivative-binary (per variables.sh:109) — not into the
   source tree under linux/build/derivative-maker. The previous
   `find . -maxdepth 6 -type f -name "*.iso"` would have missed
   everything once we got that far. Updated to
   `find "${HOME}/derivative-binary" ...`.

No image rebuild needed — this is a pure script-and-env change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

linux/build/scripts/build-inner.sh

@@ -26,6 +26,28 @@ set -euo pipefail
 : "${REPO_ROOT:?REPO_ROOT must be set}"
 : "${BUILD_DIR:?BUILD_DIR must be set}"
 
+# Explicit user_name pin.
+# derivative-maker/help-steps/variables (lines 80-93) computes user_name
+# from $SUDO_USER as its first non-empty fallback. We enter this script
+# via `sudo --preserve-env -u user --` from root, which makes sudo set
+# SUDO_USER=root (the *calling* user). Variables.sh then picks
+# user_name="root" and computes HOMEVAR=/home/root — which doesn't exist
+# (root's home is /root). The first thing that breaks under that path
+# is the aptgetopt config tee in 1100_sanity-tests:
+#     tee: /home/root/derivative-binary/30_derivative-maker.conf:
+#     No such file or directory
+# Setting user_name explicitly satisfies the first-priority check in
+# variables.sh and short-circuits the SUDO_USER fallback.
+export user_name=user
+
+# Create the binary output directory derivative-maker writes into.
+# variables.sh sets binary_build_folder_dist=$HOMEVAR/derivative-binary
+# (= /home/user/derivative-binary), and 1100_sanity-tests / later steps
+# expect it to exist. Upstream's docker-start does the equivalent
+# `mkdir --parents -- "${HOME}/derivative-binary"`; we replicate that
+# here so we don't depend on upstream's wrapper.
+mkdir -p "${HOME}/derivative-binary"
+
 # shellcheck disable=SC1091
 source "${REPO_ROOT}/linux/build/config/silvermetal-base.conf"
 
@@ -54,13 +76,14 @@ cd "${REPO_ROOT}/linux/build/derivative-maker"
     --allow-untagged    true \
     --allow-uncommitted true
 
-# derivative-maker writes into its own build/ tree; collect into BUILD_DIR.
+# derivative-maker writes its outputs into ${HOME}/derivative-binary
+# (per help-steps/variables: binary_build_folder_dist=$HOMEVAR/derivative-binary),
+# *not* into the source tree. Collect from there into BUILD_DIR.
 # Exact upstream output paths can shift between tags — keep this tolerant.
-# Anything matching *.iso under the tree is what we want.
-find . -maxdepth 6 -type f -name "*.iso" -print0 \
+find "${HOME}/derivative-binary" -maxdepth 6 -type f -name "*.iso" -print0 \
     | xargs -0 -I{} cp -av "{}" "${BUILD_DIR}/"
 
 # Manifest of file metadata that lives inside the ISO. Useful when
 # diagnosing reproducibility regressions without re-extracting.
-find . -maxdepth 6 -type f -name "*.manifest" -print0 \
+find "${HOME}/derivative-binary" -maxdepth 6 -type f -name "*.manifest" -print0 \
     | xargs -0 -I{} cp -av "{}" "${BUILD_DIR}/" 2>/dev/null || true