playground: disable cgo when executing the vet tool

Go binary installed in Docker images is not built with
CGO_ENABLED=0. "go vet" was failing for the programs that
imported packages with cgo files.

This change disables the usage of cgo files in the vet tool.

Fixes golang/go#26307.

Change-Id: I0ce117a94d34b0241d48f91cb8165ab38c47cc2d
Reviewed-by: Brad Fitzpatrick <>
diff --git a/vet.go b/vet.go
index 314fd40..b4925db 100644
--- a/vet.go
+++ b/vet.go
@@ -29,6 +29,10 @@
 	cmd := exec.Command("go", "vet", in)
+	// Linux go binary is not built with CGO_ENABLED=0.
+	// Prevent vet to compile packages in cgo mode.
+	// See #26307.
+	cmd.Env = append(os.Environ(), "CGO_ENABLED=0")
 	out, err := cmd.CombinedOutput()
 	if err == nil {
 		return &response{}, nil