message/pipeline: skip TestFullCycle on plan9-arm

This test has timed out several times recently on this builder. The
test is pretty slow even on Linux, and may be especially
filesystem-intensive.

For golang/go#49338

Change-Id: Ife2bb399b10f369f815055bb0ad44bb007f606b7
Reviewed-on: https://go-review.googlesource.com/c/text/+/380414
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

message/pipeline/pipeline_test.go

@@ -33,6 +33,9 @@ func TestFullCycle(t *testing.T) {
 	if runtime.GOOS == "android" {
 		t.Skip("cannot load outside packages on android")
 	}
+	if b := os.Getenv("GO_BUILDER_NAME"); b == "plan9-arm" {
+		t.Skipf("skipping: test frequently times out on %s", b)
+	}
 	if _, err := exec.LookPath("go"); err != nil {
 		t.Skipf("skipping because 'go' command is unavailable: %v", err)
 	}