.github/workflows: upgrade to setup-go@v2

The rest of our workflows use v2.1.4.
For reasons I do not understand, we must set GOPATH here.
Maybe the GitHub Action builds come with GOPATH already set?

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

.github/workflows/vm.yml

@@ -12,11 +12,13 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
 
     steps:
+      - name: Set GOPATH
+        run: echo "GOPATH=$HOME/go" >> $GITHUB_ENV
+
       - name: Set up Go
-        uses: actions/setup-go@v1
+        uses: actions/setup-go@v2.1.4
         with:
           go-version: 1.17
-        id: go
 
       - name: Checkout Code
         uses: actions/checkout@v1