build: prevent the entire repo to be wiped out by cgo test scripts
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.