[Feat]: Add option to `om ci` for selective package builds

[semi710]

Description

Currently, om ci builds all packages defined in the flake output.
It would be very useful to have an option to build only specific packages or exclude certain ones.

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Proposed Feature

Add support for:

• --build .#a .#b .#c – Only builds the specified packages.
• --exclude-build .#a .#b – Builds everything except the excluded packages.

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Use Case

Example scenario:

• I have an editor configuration with three packages: bare, core, and full.
• The full package includes large dependencies (e.g., TeX packages) that I don’t want to build in CI.
• Similarly, sometimes building Docker images should be skipped in CI to save time and resources.

[Allsimon]

Another use-case: I keep a flake with the NixOS configuration of a dozen machines.

One of them needs a custom kernel (not cached by nixos-hardware). 99.9999% of the time taken in CI is wasted on this kernel.
The flag --exclude-build would be greatly appreciated :)