nixos/repart-image: refactor to use mkDerivation

As a follow-up to https://github.com/NixOS/nixpkgs/pull/294096 this
should further improve the flexibility around building OS images with
systemd-repart:

* Previously the attribute set `compression` needed to be fully
  populated, including `algorithm` and `level` because
  `compression.enable` was evaluated by bash, after being interpolated
  as strings into the `buildCommand`. Now it's sufficient to pass
  `compression.enable = false` to the builder, e.g. in `overrideAttrs`,
  to disable the compression.
* Using mkDerivation allows for much more customization than the
  previously used `runCommand`, making use of phases and pre/post hooks.
  This is especially helpful for building multiple images from the same
  system configuration, e.g. to build an image `Y` based on a partially
  built raw image `X`,  by injecting a UKI that depends on `X` into a
  defered ESP.
* Before this change it was non-trivial to conduct further manipulations
  on the amended repart definitions. Now, the definitions that
  systemd-repart uses to build the image can be easily manipulated in
  `postPatch` or `preBuild`.

Aside from this, the build is now executed in the build directory, rather
than `$out`. This allows references to relative paths in the build
environment to be used, especially for `--definitions`, which previously
required an absolute path.

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