Only run REPL build on internal branches or when specific label is set

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

This will protect the REPL build workflow by only running it on forks if a specific label x8 ⚙️ build repl artefacts has been set on a pull request.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#refine-repl-build

or load it into the REPL:
https://rollupjs.org/repl/?pr=4156

[codecov]

Codecov Report

Merging #4156 (53bfd1c) into master (9c436a7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4156   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files         201      201           
  Lines        7118     7118           
  Branches     2084     2084           
=======================================
  Hits         6996     6996           
  Misses         58       58           
  Partials       64       64           

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