Running `cross` with `--metadata-path=directory/Cargo.toml` will fail to work

[nagisa]

There are a couple of work-arounds for read-ability or write-ablity of certain files in the source of cross, but they won’t work when --metadata-path is specified.

[reitermarkus]

Yeah, this is related to the caveat in https://github.com/rust-embedded/cross#caveats. If you see a way to support arbitrary directories instead of just the directory containing the Cargo.toml, a PR would be appreciated.

[Alexhuszagh]

Should be fixed as of #684.