In the followup discussion on #110906, we discovered that their original problem was because they had chosen a path under their home directory for [install] prefix, but they had not changed the default sysconfdir = "/etc". So most things installed fine, but they didn't have permission to write /etc/bash_completion.d/cargo.

Bootstrap could sanity check this when starting an install, seeing that prefix is writable by the current user but the absolute sysconfdir is not. (A relative sysconfdir goes under prefix, which should be fine.) This should also take DESTDIR into account as a further prefix on all installation paths.