cmd/cue: skip the mod_registry testscript as it's flakey

Until I've finished a change to testscript to add a `waitmatch` command.
Otherwise the CI runs fail about 2-5% of the time with:

    > exec cue mod registry ${CUE_REGISTRY} &
    > exec cue mod publish v0.0.1
    [stderr]
    cannot put module: [...]: dial tcp [::1]:41331: connect: connection refused
    [exit status 1]

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I03f15b0daa3c338302ce36beccdb0a0a526e0492
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199625
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/testdata/script/mod_registry.txtar

@@ -1,3 +1,5 @@
+skip 'TODO(mvdan): this test is racy; re-enable once we wait for the http server to be ready'
+
 # TODO: use a random unused port
 env CUE_REGISTRY=localhost:41331
 exec cue mod registry ${CUE_REGISTRY} &