cmd/golangorg: fix cloudbuild.yaml

For not having been able to run this before submitting,
I was *so* close.

For golang/go#61940.

Change-Id: Ifb14b1ec35f731e7e886956ad422f92122625a38
Reviewed-on: https://go-review.googlesource.com/c/website/+/550135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/golangorg/cloudbuild.yaml

@@ -19,7 +19,7 @@ steps:
   - name: golang
     args: ["rm", "-rf", "_wikitmp/.git"]
   - name: golang
-    args: ["cp", "-a", "_wikitmp/*", "_content/wiki"]
+    args: ["sh", "-c", "cp -a _wikitmp/* _content/wiki"]
   # Run tests.
   - name: golang
     args: ["go", "test", "./..."]