Clean-up main.yml and build-and-test.sh

[nadinekuo]

Refactored main.yml:

• Checking for concurrent CI workflows is now done using GitHub’s built-in functionality, rather than relying on external tools
• The files needed for building and testing are operated on directly by letting the workflow runner checkout various branches into directories on the cloud (these dirs are passed as args to the scripts) → no more copying around of files
• Note that the changes to main.yml are visible under my fork only, since I cannot open a PR for both master and scripts :)

Refactored build-and-test.sh:

• Instead of relying on some remote repository on Bitbucket, the script now has direct access to all files that were in that repo, as I moved all these files into the scripts branch
• All changes that were previously made using sed are now applied to the respective files

Closes issues #7 and #11