fix(linux/build): symlink /home/user/derivative-maker to checkout (M1.1 iter23)

Run #4270's Build A made it 2:40 deep — past sanity-tests, prepare-
build-machine, local-deps, into 2100_create-debian-packages — then
died on:

    + /workspace/.../genmkfile/usr/bin/genmkfile reprepro-remove
    running: dm-reprepro-wrapper remove local age-api
    + /usr/bin/dm-reprepro-wrapper: line 28:
        /home/user/derivative-maker/help-steps/pre: No such file or directory

Earlier `dm-reprepro-wrapper includedsc/includedeb` calls succeeded
because 2100_create-debian-packages invokes them by absolute path
(`$source_code_folder_dist/packages/.../developer-meta-files/usr/bin/
dm-reprepro-wrapper`) — the in-repo copy resolves help-steps/pre
relative to its own location.

`genmkfile reprepro-remove` calls `dm-reprepro-wrapper` via PATH
instead, so the system copy at /usr/bin/dm-reprepro-wrapper wins. That
copy was installed by 1500_local-deps `apt install`-ing the in-repo
developer-meta-files.deb into the silvermetal-builder image at runtime.
The .deb's intended layout assumes the matching derivative-maker
checkout lives at /home/user/derivative-maker — the upstream-blessed
path. Ours is at /workspace/SilverLABS/SilverMetal/linux/build/
derivative-maker, so the relative source() at line 28 walks off into
nowhere.

Bridge the gap with a symlink at the start of build-inner.sh:

    ln -sfn "${REPO_ROOT}/linux/build/derivative-maker" \
            /home/user/derivative-maker

That keeps our self-referential CI bind-mount topology (we still cd
into REPO_ROOT/.../derivative-maker, derivative-maker still computes
paths relative to itself), but also makes the system copy of
dm-reprepro-wrapper find help-steps/pre and friends.

Both reprepro wrappers (in-repo and system-installed) now resolve to
the same files via the symlink, so the silvermetal-reprepro-wrap.sh
PATH precedence shadow at /usr/local/bin/reprepro keeps applying to
both code paths.

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

linux/build/scripts/build-inner.sh

@@ -69,6 +69,21 @@ if [ -d /usr/share/keyrings ]; then
     done
 fi
 
+# Bridge the upstream-blessed checkout path.
+# 1500_local-deps installs the in-repo `developer-meta-files.deb` system-
+# wide. That deb ships /usr/bin/dm-reprepro-wrapper, which begins with
+#     source "${0%/*}/../../help-steps/pre"
+# resolved against the install location (/usr/bin) and a user_name-relative
+# layout that lands at /home/user/derivative-maker/help-steps/pre.
+# When 2100_create-debian-packages calls `dm-reprepro-wrapper` via PATH
+# (e.g. through `genmkfile reprepro-remove`), the system copy wins over
+# the in-repo one, and the source fails:
+#     /usr/bin/dm-reprepro-wrapper: line 28:
+#       /home/user/derivative-maker/help-steps/pre: No such file or directory
+# Make /home/user/derivative-maker resolve to our actual checkout so both
+# the in-repo and system-installed wrappers find the same support files.
+ln -sfn "${REPO_ROOT}/linux/build/derivative-maker" /home/user/derivative-maker
+
 # shellcheck disable=SC1091
 source "${REPO_ROOT}/linux/build/config/silvermetal-base.conf"