remove usage of `@actions/core`

[meesvandongen]

In my previous issue I suggested changing the documentation to reflect the environment variables, but after some testing it seems that the environment variables are not set. Moreover, setting the environment variables is not possible from a child process (nodeJS) as far as I am aware.

For that problem specifically, I would suggest moving to files; e.g. changesets-gitlab-output.json

{
  "published": true,
  "publishedPackages": [
    { "name": "@xx/xx", "version": "1.2.0" },
    { "name": "@xx/xy", "version": "0.8.9" }
  ]
}

Then, a suggestion could be added to add the file to .gitignore.

More generally, @actions packages should not be used at all in this repo since it is only meant for Github actions.

If you are fine with such changes I can look into making a PR.

[JounQin]

Thanks @meesvandongen , actually I've never used the output feature previously.

I'm fine to migrate environment variables into ~/changesets-gitlab.output.json for example.

So, PR welcome!

[meesvandongen]

I have created a minimal change on #171, but haven't tested it yet.