CI: make all workflows get Go version from go.mod

The next time we update the toolchain, all of the CI
Actions will automatically use it when go.mod is updated.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

.github/workflows/vm.yml

@@ -19,13 +19,13 @@ jobs:
       - name: Set GOPATH
         run: echo "GOPATH=$HOME/go" >> $GITHUB_ENV
 
+      - name: Checkout Code
+        uses: actions/checkout@v3
+
       - name: Set up Go
         uses: actions/setup-go@v3
         with:
-          go-version: 1.19
-
-      - name: Checkout Code
-        uses: actions/checkout@v3
+          go-version-file: go.mod
 
       - name: Run VM tests
         run: go test ./tstest/integration/vms -v -no-s3 -run-vm-tests -run=TestRunUbuntu2004