devenv: use explicit package paths for executables

[bobvanderlinden]

As a follow-up from #545 I thought there might be more instances where executable were used from the global system. This turned out to be true.

I'm not sure whether any of the currently used executables result in incompatiblities, but I thought to avoid such potential problems in the future.

I also took the liberty to align the nix and devenv-yaml calls so they look the same as calls to other packages.

I checked for usage of other external commands by using PATH="" in the top of the devenv-script and look for command not found errors.

[bobvanderlinden]

Hmm CI is failing because it tries to run:

devenv ci

with the new changes, which results in:

/nix/store/00i4b6rg45xinc8jhf4cfdamnpisn02l-devenv-profile/bin/devenv: line 228: /nix/store/isia7synl0mdbcvfwav5ywnrwmwbwvxv-source/bin/nix: No such file or directory

There seems to be a mixup of src/devenv.nix with an older flake.nix. This is because src/devenv.nix is imported from other places than just /flake.nix.

I have now made sure the original call signature is used for src/devenv.nix, so it stays compatible. I did the resolve nix to nix.packages.${pkgs.stdenv.system}.nix inside src/devenv.nix instead of inside /flake.nix.

[bobvanderlinden]

The python-poetry test failure should've been resolved by #552. It seemed to be unrelated to this PR.