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/go_generate.yml

@@ -18,16 +18,16 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - name: Set up Go
-        uses: actions/setup-go@v3
-        with:
-          go-version: 1.19
-
       - name: Check out code
         uses: actions/checkout@v3
         with:
           fetch-depth: 0
 
+      - name: Set up Go
+        uses: actions/setup-go@v3
+        with:
+          go-version-file: go.mod
+
       - name: check 'go generate' is clean
         run: |
           if [[ "${{github.ref}}" == release-branch/* ]]