Need to detect both `source` and `.` when installing and uninstalling rustup.

[kinnison]

In #2616 we changed from source to . in order to be more compatible with non-bash shells. We should probably use source for bash and . for POSIX sh if we can separate the two, or else just use . but detect both kinds so that we don't end up with double invocations and/or failing to remove an invocation on uninstallation as per #2616 (comment)