CI: Always upload package cache even if overall build fails

When we don't trigger CI for more than 1 week, or if the
gettext/libsodium packages were updated, CI needs to re-generate the
artifacts and cache them. However, if the overall build fails later due
to unrelated test failures, previously it would stop the cache from
being uploaded at all, making iterating on fixing the test failure quite
painful as we need to rebuild the artifact every run until it passes.
This change now makes it so we always upload the cache as long as the
package was built correctly, so that CI runs after the first failure
will be much smoother.

Author: ychin

.github/actions/universal-package/action.yml

@@ -60,14 +60,15 @@ runs:
 
         echo '::endgroup::'
 
-    - name: Cache keg
-      id: cache-keg
-      uses: actions/cache@v4
+    - name: Restore keg cache
+      id: cache-keg-restore
+      uses: actions/cache/restore@v4
       with:
         path: ${{ steps.setup-formula.outputs.brew_prefix }}/Cellar/${{ inputs.formula }}
         key: ${{ inputs.formula }}-homebrew-cache-custom-unified-prefix${{ steps.setup-formula.outputs.brew_prefix }}-xcode${{ steps.setup-formula.outputs.xcode_version }}-${{ hashFiles(format('{0}.rb', inputs.formula)) }}
 
     - name: Install formula
+      id: install-formula
       shell: bash
       run: |
         echo '::group::Install formula'
@@ -100,3 +101,16 @@ runs:
         done
 
         echo '::endgroup::'
+
+    - name: Save keg cache
+      id: cache-keg-save
+      uses: actions/cache/save@v4
+      # We always save the generated artifact even if the whole run
+      # fails due to other issues. This helps debugging build
+      # failure issues faster if the cache doesn't already exist as
+      # we want subsequent runs to not have to re-generate this
+      # artifact again.
+      if: always() && steps.cache-keg-restore.outputs.cache-hit != 'true' && steps.install-formula.conclusion == 'success'
+      with:
+        path: ${{ steps.setup-formula.outputs.brew_prefix }}/Cellar/${{ inputs.formula }}
+        key: ${{ steps.cache-keg-restore.outputs.cache-primary-key }}