[CI]: Integration test for the init.lua bootstrapping script

[vhyrro]

With #281 being merged, it would be nice to have an integration test for the bootstrapping script (not using nix). It could be a matrix over all platforms, replacing the existing Windows test.

Required steps:

• init.lua with the bootstrapping logic
  • CI extracts the script from the README to retrieve the bootstrapping script
• Start neovim
• Try to run a :Rocks command