ci: consolidate release.yml into single checkout step

The two-step pattern (sparse-checkout RELEASE_NOTES.md, then full
checkout) left sparse-checkout config active on the workspace, so the
second checkout still only restored RELEASE_NOTES.md. Docker build
then failed with "open Dockerfile: no such file or directory".

Since both RELEASE_NOTES.md and the full source are needed in the same
job, one full checkout is simpler and correct.

.gitea/workflows/release.yml

@@ -13,13 +13,7 @@ jobs:
   release:
     runs-on: ubuntu-latest
     steps:
-      - name: Fetch RELEASE_NOTES.md only
-        uses: actions/checkout@v4
-        with:
-          sparse-checkout: RELEASE_NOTES.md
-          sparse-checkout-cone-mode: false
-
-      - name: Checkout full repo for Docker build
+      - name: Checkout repo
         uses: actions/checkout@v4
 
       - name: Extract version from tag