Previously, it was not possible to inject PATH entries between profiles
and the “default system” PATH entries. This confounds adding, e.g.
Homebrew on aarch64’s non-standard prefix as higher priority than the
builtin system paths, but lower than Nix profiles.
This is a backwards-incompatible change for some users, but should only
be so in the case a user used `mkOrder` with a value between 1000 (the
default priority) and 1200. Value of 1200 chosen as the same delta from
the default as just below in `environment.profiles` (which uses 800),
and mkAfter is 1500 so will still go after this.
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