Explicitly set GOPATH

Although Github Actions provides a Go action, that action doesn't set up the environment for use with Go.

actions/setup-go#14 suggests that this is broken-as-intended.

The two workarounds suggested there are using the Go container image to run tests; or to explicitly set up the environment like this.

.github/workflows/go.yml

@@ -12,6 +12,12 @@ jobs:
         go-version: 1.13
       id: go
 
+    - name: setup env
+      run: |
+        echo "::set-env name=GOPATH::$(go env GOPATH)"
+        echo "::add-path::$(go env GOPATH)/bin"
+      shell: bash
+
     - name: Check out code into the Go module directory
       uses: actions/checkout@v1