kola: Skip cl.sysext.fallbackdownload for release builds

We currently have no way to switch the update pub key in the initrd to
be a dev key (or vice versa). Thus the test cl.sysext.fallbackdownload
failed because based on the key either the update server or bincache is
tried.
Skip cl.sysext.fallbackdownload for release builds at least temporarily.
Later we might use a kernel cmdline setting or similar to switch the
key to a dev key.

kola/tests/update/update.go

@@ -452,7 +452,12 @@ func sysextFallbackDownload(c cluster.TestCluster) {
 	client := &http.Client{
 		Timeout: time.Second * 60,
 	}
-	// For simplicity, only support bincache (which is also used for release builds), the test will be skipped on GitHub PRs
+	// Check that we are on a dev build. Overwriting the pub key with a bind mount won't work for the initrd.
+	keySum := string(c.MustSSH(m, `md5sum /usr/share/update_engine/update-payload-key.pub.pem | cut -d " " -f 1`))
+	if keySum != "7192addf4a7f890c0057d21653eff2ea" {
+		c.Skip("Test skipped, only dev builds are supported")
+	}
+	// For simplicity, only support bincache, the test will be skipped on GitHub PRs and release builds.
 	// The URL comes from bootengine:dracut/99setup-root/initrd-setup-root-after-ignition where it would be flatcar_test_update-oem-qemu.gz
 	// but here we instead test for version.txt to still run and fail the test if that file is missing for unknown reasons
 	reply, err := client.Head(fmt.Sprintf("https://bincache.flatcar-linux.net/images/%s/%s/version.txt", arch, version))