rustc: Avoid /tmp/ in graphviz writing

This issue was reported to security@rust-lang.org by Sebastien Marie following
our recent [security advisory][1]. Because `/tmp` is typically globally writable
it's possible for one user to place symlinks in `/tmp` pointing to files in
another user's directories, causing `rustc` to overwrite the contents of
innocent files by accident.

This patch instead defaults the output path here to the cwd which should avoid
this issue.

[1]: https://blog.rust-lang.org/2018/07/06/security-advisory-for-rustdoc.html

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