nix3-profile automatically migrates any profile its used on to its style
of profile -- the ones with manifest.json instead of manifest.nix. On
non-NixOS systems, Nix is conventionally installed to the profile at
/nix/var/nix/profiles/default, so if a user passed that to `--profile`
of `nix profile`, then it would break upgrade-nix from ever working
again, without recreating the profile.
This commit fixes that, and allows upgrade-nix to work on either kind of
profile.
Fixes#16.
Change-Id: I4c49b1beba93bb50e8f8a107edc451affe08c3f7
