installer: Use PowerShell's "tee" to log mingw-get's output to a file

The "%{ \"$_\" }" magic is required to suppress PowerShell's
NativeCommandError output, see [1]. Note that Powershell outputs Unicode
(UCS-2 Little Endian) encoded files.

[1] http://stackoverflow.com/a/12866669/1127485

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>