Following the parallelization of some tests, a race condition can
occur in testcarchive, testshared and testcshared.

In some cases, it can result in the go env GOROOT command returning
corrupted data, which are then passed to a rm command.

Make the shell script more robust by not trusting the result of
the go env GOROOT command. It does not really fix the issue, but
at least prevent the entire repository to be deleted.

Updates #13789

Change-Id: Iaf04a7bd078ed3a82e724e35c4b86e6f756f2a2f
Reviewed-on: https://go-review.googlesource.com/18173
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>