fix(ci): remove build-check job from CI workflow (#66)

* ci: remove build-check job from CI workflow

The cross-platform build verification is redundant in CI since the
build and release pipeline (build.yml) already performs full builds
on every release. This reduces CI runner time and costs.

* ci: restrict build-check to Linux PRs and pin R2 action to v1.4

- Build-check now only runs on pull requests (skipped on main merge)
  with only Linux/amd64 enabled; macOS and Windows are commented out
  since the Build & Publish workflow handles all platforms on release.
- Pin ryand56/r2-upload-action from floating v1 tag to v1.4 to fix
  the upload failure seen in the v1.1.0 release build.

.github/workflows/build.yml

@@ -176,7 +176,7 @@ jobs:
             artifacts/release/yellowjacket-windows-amd64.exe
 
       - name: Upload to R2
-        uses: ryand56/r2-upload-action@v1
+        uses: ryand56/r2-upload-action@v1.4
         with:
           r2-account-id: ${{ secrets.R2_ACCOUNT_ID }}
           r2-access-key-id: ${{ secrets.R2_ACCESS_KEY_ID }}