ci: run the musl build & test under mkosi with a postmarketOS tools tree

Drop the standalone Unit-tests (musl) workflow that ran on an Alpine sandbox
spun up by jirutka/setup-alpine, and merge it into unit-tests.yml as a new
build-musl job that provisions a postmarketOS tools tree via mkosi and runs
the meson build + test suite through 'mkosi box'. postmarketOS is musl-native,
so the musl-gcc / -idirafter /usr/include wrappers the Fedora tools tree
needed are gone; the linter.yml's own musl build step also goes away since
the unit-tests workflow now covers it (and tests it).

postmarketOS doesn't ship a downstream systemd packaging spec, so the new
tools tree config in mkosi.tools.conf/mkosi.conf.d/postmarketos.conf does not
set PrepareScripts and lists build deps manually. mkosi.sync now early-exits
when PKG_SUBDIR is unset so the missing pkgenv entry doesn't trip set -u.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/linter.yml

@@ -87,14 +87,3 @@ jobs:
 
       - name: Run coccinelle checks
         run: mkosi box -- meson test -C build --suite=coccinelle --print-errorlogs --no-stdsplit
-
-      - name: Build with musl
-        run: |
-          mkosi box -- \
-            env \
-            CC=musl-gcc \
-            CXX=musl-gcc \
-            CFLAGS="-idirafter /usr/include" \
-            CXXFLAGS="-idirafter /usr/include" \
-              meson setup -Dlibc=musl -Ddbus-interfaces-dir=no musl
-          mkosi box -- ninja -C musl