Build only spec files that were changed in the push

Add a 'changed' job that determines which .spec files were modified
using git diff between push before/after SHAs. The build job now
uses a dynamic matrix (fromJSON) so only changed specs are built.
For workflow_dispatch, all specs are still built.

.forgejo/workflows/build-rpms.yml

@@ -7,15 +7,32 @@ on:
   workflow_dispatch:
 
 jobs:
+  changed:
+    runs-on: almalinux-10
+    outputs:
+      matrix: ${{ steps.set.outputs.matrix }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Determine changed specs
+        id: set
+        run: |
+          if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
+            python3 -c "import json; print(json.dumps({'spec': ['sonic-interface-libraries.spec','sonic-win.spec','sonic-workspace.spec']}))"
+          else
+            CHANGED=$(git diff --name-only ${{ github.event.before }} ${{ github.sha }} | grep '\.spec$' || true)
+            python3 -c "import json, sys; print(json.dumps({'spec': sys.argv[1].split()}))" "$CHANGED"
+          fi >> "$GITHUB_OUTPUT"
+
   build:
+    needs: changed
     runs-on: almalinux-10
     strategy:
       fail-fast: false
-      matrix:
+      matrix: ${{ fromJSON(needs.changed.outputs.matrix) }}
-        spec:
-          - sonic-interface-libraries.spec
-          - sonic-win.spec
-          - sonic-workspace.spec
 
     steps:
       - name: Checkout