gopls/internal/lsp/filecache: skip TestConcurrency on plan9 builder

Skip due to golang/go#58748. We do not have resources to investigate
this failure.

Updates golang/go#59981
Fixes golang/go#58748

Change-Id: I5d695fbeab0b29cedf481ca6b2311bd8c399c4a5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/492495
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: findleyr

gopls/internal/lsp/filecache/filecache_test.go

@@ -60,6 +60,9 @@ func TestBasics(t *testing.T) {
 
 // TestConcurrency exercises concurrent access to the same entry.
 func TestConcurrency(t *testing.T) {
+	if os.Getenv("GO_BUILDER_NAME") == "plan9-arm" {
+		t.Skip(`skipping on plan9-arm builder due to golang/go#58748: failing with 'mount rpc error'`)
+	}
 	const kind = "TestConcurrency"
 	key := uniqueKey()
 	const N = 100 // concurrency level