dotnet: ensure sources are cleaned up after install

[carlocab]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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This leaves behind 50GB in build artefacts without this change.

[cho-m]

Will also need the self-hosted Linux as GitHub runners don't have enough disk space to build dotnet.

And probably want to recreate it before run since there may still be data from last build & we only have a 100G limit.