cmd/go: fix TestNewReleaseRebuildsStalePackagesInGOPATH

CL 307818 added a package that the runtime depends on, but didn't
update the list of runtime dependencies in this test.

This should fix the longtest builder failures.

Change-Id: I5f3be31b069652e05ac3db9b1ce84dd5dfe0f66f
Reviewed-on: https://go-review.googlesource.com/c/go/+/308469


Trust: Austin Clements <austin@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>