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.
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