Fail clearly if the Nix installer leaves no profile script

Fail clearly if the Nix installer leaves no profile script Found by a stubbed harness exercising bootstrap.sh's fresh-machine paths, which had never run: with no /nix present, the script sourced $NIX_PROFILE_SH unconditionally after installing, so a missing profile produced a bare "No such file or directory" partway through the bootstrap. Now it checks first and exits with an instruction to open a new terminal and re-run. The source is also wrapped in set +u/-u like the one at the top of the script, since the profile is not written against set -u. Scenarios covered: fresh machine with a working installer (completes all four steps in order), fresh machine where the installer leaves no profile (clean failure), already-configured machine, and `bootstrap.sh work` (switch targets #work). shellcheck is clean.