Currently "/usr/bin/env python" is rewritten to "/nix/store/.../env
python". That doesn't really improve anything because the interpreter
still have to be located in $PATH at runtime. The result is that many
nix package expressions do .../bin/env fixup themselves.
Instead of everyone having to do this patching locally, add the
functionality to the standard environment patchShebangs function so that
everyone can benefit.
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