ci: upload the Windows/CUDA viewer installer as a build artifact

The Windows installer was only uploaded on tagged releases, so testing an
arbitrary branch build meant cutting a tag. Also upload the CUDA installer as a
workflow artifact on every run, with a short 3-day retention (it is a throwaway
test build). Only the cuda variant, since that is the one we test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build_and_test.yml

@@ -97,6 +97,16 @@ jobs:
           call "%VSPATH%\VC\Auxiliary\Build\vcvars64.bat"
           cd build
           cpack
+      - name: Upload CUDA installer as build artifact
+        # Keep the Windows/CUDA installer downloadable from every run (not just tagged releases) so it
+        # can be grabbed and tested. Short retention - it is a throwaway test build, not a release.
+        if: matrix.variant == 'cuda'
+        uses: actions/upload-artifact@v3
+        with:
+          name: jfjoch-viewer-windows-cuda-installer
+          path: build/jfjoch-viewer-*-win64-cuda*.exe
+          retention-days: 3
+          if-no-files-found: error
       - name: Upload installer to release
         if: github.ref_type == 'tag'
         shell: powershell