fix(gcp): satisfy lint in TestMissingStaticTokenFallsBackToADC

Replace os.Setenv-in-cleanup with t.Setenv (the cleanup it registers is
sufficient for restoration) and add the blank line wsl_v5 wants before
the unsetenv call.

Author: james00012

modules/gcp/static_token_test.go

@@ -33,15 +33,13 @@ func TestStaticTokenShortCircuitsCredentialLookup(t *testing.T) {
 }
 
 // Without the static token env var, constructors must fall back to ADC, which
-// fails when the credentials file is poisoned.
+// fails when the credentials file is poisoned. t.Setenv records the original
+// value and registers cleanup; the subsequent Unsetenv triggers the "not set"
+// branch in getStaticTokenSource for the duration of the test.
 func TestMissingStaticTokenFallsBackToADC(t *testing.T) {
-	orig, had := os.LookupEnv("GOOGLE_OAUTH_ACCESS_TOKEN")
+	t.Setenv("GOOGLE_OAUTH_ACCESS_TOKEN", "")
 	require.NoError(t, os.Unsetenv("GOOGLE_OAUTH_ACCESS_TOKEN"))
-	t.Cleanup(func() {
-		if had {
-			os.Setenv("GOOGLE_OAUTH_ACCESS_TOKEN", orig)
-		}
-	})
+
 	t.Setenv("GOOGLE_APPLICATION_CREDENTIALS", "/nonexistent/credentials.json")
 
 	_, err := gcp.NewCloudBuildServiceE(t)