Fix vm ci tests clogging in fork repository pull request

fix: https://github.com/tailscale/tailscale/issues/5771
Signed-off-by: Aoang <aoang@x2oe.com>

.github/workflows/vm.yml

@@ -14,7 +14,7 @@ jobs:
   ubuntu2004-LTS-cloud-base:
     runs-on: [ self-hosted, linux, vm ]
 
-    if: "!contains(github.event.head_commit.message, '[ci skip]')"
+    if: "(github.repository == 'tailscale/tailscale') && !contains(github.event.head_commit.message, '[ci skip]')"
 
     steps:
       - name: Set GOPATH