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/linux32.yml

@@ -19,16 +19,15 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
 
     steps:
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v3
 
     - name: Set up Go
       uses: actions/setup-go@v3
       with:
-        go-version: 1.19
+        go-version-file: go.mod
       id: go
 
-    - name: Check out code into the Go module directory
-      uses: actions/checkout@v3
-
     - name: Basic build
       run: GOARCH=386 go build ./cmd/...