Add Linux build workflow and remove ISO file in order to execute the build on GitHub Actions

[LegendaryGuard]

That builds everytime you push a commit, so the action runs. After building, you get the artifact (the compiled ISO file).

Go to Actions:

Select a workflow (it will display the commit message, even if the workflow is running):

And download the artifact (it's compressed with .zip):

Keep in mind, the build workflow on artifact storage retention, so it expires after x days. The expiration/retention days option is made by storage saving. I set the expiration to 7 days (one week), it can be changed on this line: https://github.com/pac-ac/osakaOS/pull/21/files#diff-5c3fa597431eda03ac3339ae6bf7f05e1a50d6fc7333679ec38e21b337cb6721R29

EDIT: In case you don't want commits from contributors, you can copy my changes and make them yours. It would be cool to give some credit, but really I don't care if you do/don't. ;)