Misc Rust/Cargo improvements (incl. use global cargo cache)

https://cgit.alxu.ca/tor.git/log/?h=misc-rust

for discussion (since nobody cared on #tor-dev): should we use the global cargo cache? I think most C+Rust projects still use the global cache. I tried searching GitHub (https://github.com/search?q=%22CARGO_HOME%22+extension%3Aam&type=Code). I found that tor is the only project that does not. for users who do not care, using the global cache will save download time and bandwidth on repeat builds, and for those who do care, my patch prints a warning so they will know. (maybe it should be downgraded to NOTICE?)