Apply buildFromSdist *after* settings overlay

[srid]

buildFromSdist appears to behave oddly so as to invalid user's settings (jailbreak, check, etc.) from the next overlay.

The solution is to create a 3rd overlay, dedicated to applying buildFromSdist and add this overlay after packages and settings overlay.

Resolves #238 ... in particular, I can build @johnhampton's repo. PR is against #251 (which contains the bug repro) for easier review.

[srid]

This breaks emanote,

error: output '/nix/store/dkkqlwi0ws49qnfsx6z6bnwknx80hnwl-emanote-source-1.3.13.0' is not allowed to refer to the following paths:
         /nix/store/057aqq4v14y27d0hg4bzwmvb94rzj894-pandoc-3.0.1
         /nix/store/2jjbsfmnqcq9750d1mzvif4qadgl74bz-pandoc-types-1.23.1
         /nix/store/qi7ls0snf355p9n5qc9mj6z7qxgkwzmf-warp-3.3.25
         /nix/store/r8zs7h9dnxapndfda463k9rw78vlwpbq-pandoc-3.1.11

❯ ls /nix/store/dkkqlwi0ws49qnfsx6z6bnwknx80hnwl-emanote-source-1.3.13.0
 emanote-1.3.13.0.tar.gz   lib

[srid]

@roberth I'm gonna timebox the debugging/fix for this. If I can't find anything that works in time, I'm gonna just make buildFromSdist a settings option and disable it by default. Correctness > features.

[srid]

Closing in favour of #253