Fix an issue where a shared storage path on Linux, that maps via two-way
variables to a drive root on Windows, caused problems with the path
translation system.
Windows paths that consist only of a drive letter (`F:`) cannot just be
concatenated to a relative path, as that will result in `F:path\to\file`,
which is still a relative path of sorts. This is now handled correctly,
and should result in `F:\path\to\file`.
This fixes#104237.
dae5b1a571