gimp: Hardcode interpreter paths

The E (extension) line in pygimp.interp prints the following warnings:

GIMP-Warning: Bad interpreter referenced in interpreter file /nix/store/5c91dah385gn70blsn6i5rzncqj3ks0y-gimp-with-plugins-2.10.18/lib/gimp/2.0/interpreters/pygimp.interp: python2
GIMP-Warning: Bad binary format string in interpreter file /nix/store/5c91dah385gn70blsn6i5rzncqj3ks0y-gimp-with-plugins-2.10.18/lib/gimp/2.0/interpreters/pygimp.interp

This means that the py extension will not be registered when python2 is not in PATH. To prevent this, let’s hardcode the absolute python2 path.

https://github.com/NixOS/nixpkgs/issues/60937#issuecomment-653652093

Closes: https://github.com/NixOS/nixpkgs/issues/60937