actions: use actions/setup-go@v2

setup-go@v2 defaults to only using stable go versions, so a go version
of "1.x" will never pull in an unstable release (as is happening with
setup-go@v1).

.github/workflows/tests.yml

@@ -34,7 +34,7 @@ jobs:
       with:
         access_token: ${{ github.token }}
 
-    - uses: actions/setup-go@v1
+    - uses: actions/setup-go@v2
       with:
         go-version: ${{ matrix.go-version }}
     - uses: actions/checkout@v2