ghactions: Automatically upload release artifacts

This makes use of https://github.com/softprops/action-gh-release to
automatically upload binaries to new releases.

.github/workflows/go.yml

@@ -3,6 +3,8 @@ name: Go
 on:
   push:
     branches: [ main ]
+    tags:
+      - 'v*'
   pull_request:
     branches: [ main ]
 
@@ -33,6 +35,17 @@ jobs:
         name: gvisor-tap-vsock-binaries
         path: bin/*
 
+    - name: Release
+        uses: softprops/action-gh-release@v1
+        if: startsWith(github.ref, 'refs/tags/')
+        with:
+          files: |
+            bin/gvforwarder
+            bin/gvproxy-darwin
+            bin/gvproxy-linux
+            bin/gvproxy-windows.exe
+            bin/qemu-wrapper
+
   tests:
     runs-on: macos-latest # Only Mac runners support nested virt
     needs: build # Don't bother testing if cross arch build fails