Bootstrap should support build directories within an unrelated git repository

[jyn514]

#98483 recently broke the case where the source directory is built from a git clone, but the build directory is inside a different, unrelated git clone. Bootstrap shouldn't have opinions on where the build directory is; we should fix it.

We can detect this case by seeing if config.src.join("src").join("stage0.json") is present; if not, we guessed the source directory incorrectly and we should fallback to the old logic using CARGO_MANIFEST_DIR.

Originally posted by @cr1901 in #98483 (comment)