In addition to minor changes for Windows, this forces Windows Python to
be used for all Windows builds (instead of using Windows Python only for
pc-windows-msvc builds).
We already redirect stdout, but the problem is that (at least on Windows/MSYS2), the `which` command tends to output things to stderr when failing:
```
$ ./mach build -d
which: no python2.7 in (/usr/local/bin:/usr/bin:/bin:/opt/bin:/c/Windows/System32:/c/Windows:/c/Windows/System32/Wbem:/c/Windows/System32/WindowsPowerShell/v1.0/:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/c/Program Files/Java/jdk1.8.0_66/bin:/c/Python27:/c/Python27/Scripts)
```
This PR silences this noise.
