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/unit-tests.yml

@@ -53,3 +53,55 @@ jobs:
           sudo --preserve-env=GITHUB_ACTIONS,CI .github/workflows/unit-tests.sh SETUP
       - name: Build & test
         run: sudo --preserve-env=GITHUB_ACTIONS,CI .github/workflows/unit-tests.sh RUN_${{ matrix.run_phase }}
+
+  build-musl:
+    name: Build & test (musl, postmarketOS)
+    runs-on: ubuntu-24.04
+    concurrency:
+      group: ${{ github.workflow }}-musl-${{ github.ref }}
+      cancel-in-progress: true
+    steps:
+      - name: Repository checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
+        with:
+          persist-credentials: false
+
+      - uses: systemd/mkosi@444d247d1a1328bcfb84a945e84959bd4bd0e02d
+
+      - name: Install apk
+        # ubuntu-24.04 doesn't package apk, so fetch the official statically-linked binary from
+        # upstream so mkosi can use it to populate the postmarketOS tools tree.
+        run: |
+          sudo curl -fsSL -o /usr/local/bin/apk \
+              https://gitlab.alpinelinux.org/api/v4/projects/5/packages/generic/v3.0.6/x86_64/apk.static
+          echo 'f1489e05bace7d7dd0a687fcd38d50b585ac660af4231668b123649bef3718c4  /usr/local/bin/apk' | sha256sum --check
+          sudo chmod +x /usr/local/bin/apk
+
+      - name: Build tools tree
+        run: |
+          tee mkosi/mkosi.local.conf <<EOF
+          [Build]
+          ToolsTreeDistribution=postmarketos
+          EOF
+
+          tee mkosi/mkosi.tools.conf/mkosi.local.conf <<EOF
+          [Distribution]
+          RepositoryKeyFetch=yes
+          EOF
+
+          mkosi -f box -- true
+
+      - name: Build
+        run: |
+          mkosi box -- \
+              meson setup \
+                  --werror \
+                  -Dtests=unsafe \
+                  -Dslow-tests=true \
+                  -Dfuzz-tests=true \
+                  -Dlibc=musl \
+                  build
+          mkosi box -- ninja -v -C build
+
+      - name: Test
+        run: mkosi box -- meson test -C build --print-errorlogs --no-stdsplit --quiet