Fix nix-drv-mode display

[daanturo]

Currently nix-drv-mode prints an unsightly "warning: 'show-derivation' is a deprecated alias for 'derivation show'" at the top of the file. The output is also all squeezed into a single line.

This PR solves those problem, plus using call-process instead of the SHELL-depedent shell-command-to-string.

For example

before

warning: 'show-derivation' is a deprecated alias for 'derivation show'
{"derivations":{"95bv7fc77k99ngjnbm5idzs2zysi7y9n-emacs-30.2.drv":{}},"version":4}

after

{
  "derivations": {
    "95bv7fc77k99ngjnbm5idzs2zysi7y9n-emacs-30.2.drv": {
      // ...
    }
  },
  "version": 4
}

[daanturo]

My apology, I didn't notice duplication of #208. I still think that PR should also add always --pretty and guard replacement to file visiting buffers only.