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>

tools/fetch-mkosi.py

@@ -15,7 +15,9 @@ from pathlib import Path
 URL = 'https://github.com/systemd/mkosi'
 BRANCH = 'main'  # We only want to ever use commits on upstream 'main' branch
 CONFIG = Path('mkosi/mkosi.conf')
-WORKFLOWS = [Path('.github/workflows') / f for f in ['mkosi.yml', 'coverage.yml', 'linter.yml']]
+WORKFLOWS = [
+    Path('.github/workflows') / f for f in ['mkosi.yml', 'coverage.yml', 'linter.yml', 'unit-tests.yml']
+]
 
 
 def parse_args():