github: install APK for testing postmarketOS

Since https://github.com/systemd/mkosi/commit/b9d4c0623975d75788627c114147be394939c7cb,
APK is not installed by default. Install it when using postmarketOS.

(cherry picked from commit 70e73a0b3fddf6fba860f7d255f7b886b02413ac)

.github/workflows/mkosi.yml

@@ -94,6 +94,9 @@ jobs:
         with:
           persist-credentials: false
       - uses: systemd/mkosi@ecf7f5702bb2052b7a69cab3fe7d37bc3b9ad77b
+        with:
+          # Bootstrapping a postmarketOS tools tree on the Ubuntu host needs apk
+          apk: ${{ matrix.distro == 'postmarketos' }}
 
       # Freeing up disk space with rm -rf can take multiple minutes. Since we don't need the extra free space
       # immediately, we remove the files in the background. However, we first move them to a different location

.github/workflows/unit-tests.yml

@@ -75,6 +75,9 @@ jobs:
           persist-credentials: false
 
       - uses: systemd/mkosi@ecf7f5702bb2052b7a69cab3fe7d37bc3b9ad77b
+        with:
+          # Bootstrapping a postmarketOS tools tree on the Ubuntu host needs apk
+          apk: true
 
       - name: Build tools tree
         run: |