Allow for in-session changes to $PATH

This change is intended to retain any changes made to `$PATH` between when the `$GVM_PATH_BACKUP` snapshot is taken and any subsequent `gvm use` invocations.  The resetting of `$PATH` is a behavior which has caused repeated confusion for myself and friends, and has recently become a source of pain on Travis, where changes to `$PATH` in the global env matrix are lost.  I realize I'm adding a wad of `tr` pipe nonsense, so I'm totally prepared to be shot down, at which point I'll try again (assuming there's feedback).