Rollup merge of #141789 - ferrocene:hoverbear/exclude-cargo-home-from-in-tree-consideration, r=clubby789

Exclude `CARGO_HOME` from `generate-copyright` in-tree determination

On Ferrocene, we noticed that in our releases the out-of-tree notices were not being included. When `x.py run generate-copyright` was ran on local development machines, it worked fine.

After some investigations ``@tshepang`` and I determined that the problem was that the cargo registry (located in `CARGO_HOME`) started with the source directory on CI jobs, and was being excluded by this line:

https://github.com/rust-lang/rust/blob/15825b7161f8bd6a3482211fbf6727a52aa1166b/src/tools/generate-copyright/src/cargo_metadata.rs#L85-L88

In Ferrocene's `run.sh` we set `CARGO_HOME` to be `build/cargo-home`: https://github.com/ferrocene/ferrocene/blob/96a45dd9a18c6e54d3cd81750a78fe459fa48af0/ferrocene/ci/run.sh#L34-L46 which caused this issue.

This PR passes the `CARGO_HOME` variable to the `generate-copyright` tool and expands the consideration of in-tree-ness to be aware of `CARGO_HOME`. It is an upstreaming of ferrocene/ferrocene#1491.

## Testing

Run `CARGO_HOME=build/cargo-home ./x.py run generate-copyright` on `master`, then check `build/host/doc/COPYRIGHT` and look for out of tree dependencies (at the bottom).

Then, try running the same command in this branch.

Author: GuillaumeGomez