cmd/dist: do not reinstall runtime/cgo with -tags lldb on ios builders

cmd/dist: do not reinstall runtime/cgo with -tags lldb on ios builders

The cache will take care of keeping go test -tags lldb fast.
Installing runtime/cgo this way just makes all the checkNotStale
tests think runtime/cgo is out of date.

Should fix ios builders.

Fixes #22509.

Change-Id: If092cc4feb189eb848b6a22f6d22b89b70df219c
Reviewed-on: https://go-review.googlesource.com/76020
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>