chore(deps): update workflow action versions (#792)

## Summary
- `actions/checkout`: v4/v5 → v6
- `actions/setup-go`: v5 → v6
- `docker/build-push-action`: v5 → v6

All other actions (`goreleaser/goreleaser-action@v6`, `docker/setup-qemu-action@v3`, `docker/setup-buildx-action@v3`, `docker/login-action@v3`, `crazy-max/ghaction-import-gpg@v6`) were already at their latest major versions.

No breaking changes affect the current workflow configurations. The main changes in the updated actions are Node.js 24 runtime upgrades and minor feature additions.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/792
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-committed-by: silverwind <[email protected]>

.gitea/workflows/release-nightly.yml

@@ -17,10 +17,10 @@ jobs:
   goreleaser:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version-file: "go.mod"
       - name: goreleaser
@@ -52,7 +52,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0 # all history for all branches and tags