BuilderSourceFile: Delete destination file

Sometimes you want to replace an existing file, like a config.sub,
and sometimes that existing config.sub does not have the writable
bits set. Just delete the file if we intend to replace it anyway.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f