Find bin paths at runtime

[DogLooksGood]

In NixOS and Guix, the executables do not exist globally. So it's better to find executables at runtime instead of defining them at startup.

[nemethf]

If the binaries are not in the PATH, executable-find won't be able to find them, and with this pull request, the user won't be able to specify their locations.

[kwrooijen]

This PR seems to break backwards compatibility, as well as customization, as @nemethf stated. It sounds to me like the best option would be for the small group of NixOS / Guix users to specify the path by changing cargo-process--custom-path-to-bin in their config.

[DogLooksGood]

It makes sense!

I just updated the PR. Now these two variables support nil, which let it find paths at runtime. How do you guys think?

[nemethf]

Running these cargo-process--find-*-path defuns at every shell-command seems unnecessarily wasteful to me.

[DogLooksGood]

With the latest update, the default behavior is unchanged. The runtime detection is only enabled when these variables are set to nil manually.