ci: only run the renovate config check if the config changes

Otherwise it's just wasting time.

.forgejo/workflows/check.yml

@@ -14,14 +14,6 @@ jobs:
           nix --version
           # shellcheck disable=SC2016
           nix build --print-build-logs '.#checks.x86_64-linux.${{ matrix.check }}'
-  check-renovaterc:
-    runs-on: nixos
-    steps:
-      - uses: "https://git.salame.cl/actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683" # v4
-      - name: Validate renovaterc.json
-        run: |
-          nix --version
-          nix shell nixpkgs#renovate --command renovate-config-validator
   report-size:
     runs-on: nixos
     needs: check

.forgejo/workflows/renovate.yml

@@ -0,0 +1,14 @@
+on:
+  push:
+    paths:
+      # only run if the renovate config changed
+      - renovate.json
+jobs:
+  check-renovaterc:
+    runs-on: nixos
+    steps:
+      - uses: "https://git.salame.cl/actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683" # v4
+      - name: Validate renovaterc.json
+        run: |
+          nix --version
+          nix shell nixpkgs#renovate --command renovate-config-validator