Auto merge of #9378 - ehuss:help-windows, r=alexcrichton
Auto merge of #9378 - ehuss:help-windows, r=alexcrichton

Handle man pages better on Windows.

If a user has `man` installed on Windows via msys/mingw/etc, then `cargo help <subcommand>` would fail with `No manual entry for C:\Users\User\AppData\Local\Temp\cargo-manlSKwTQ`.  This is because the cygwin universe does not handle windows-style paths and does not auto-translate in this scenario.

The solution here is to run the command from within the temp directory and use a relative path which *should* work on all platforms and environments. I tested on windows (powershell, cmd, and mingw), macos, and linux.

Fixes #9197

rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,rustyhorde/cargo