Unable to run tests with `subst` drive letters

Encountered by @H-G-Hristov in #4611.

Possibly related to the codepaths that I changed in #4395.

Confirmed by @AlexGuteniev on Discord, quoting his repro:

C:\Project\STL\out\x64>subst /?
Associates a path with a drive letter.

SUBST [drive1: [drive2:]path]
SUBST drive1: /D

  drive1:        Specifies a virtual drive to which you want to assign a path.
  [drive2:]path  Specifies a physical drive and path you want to assign to
                 a virtual drive.
  /D             Deletes a substituted (virtual) drive.

Type SUBST with no parameters to display a list of current virtual drives.

C:\Project\STL\out\x64>subst D: C:\Project

C:\Project\STL\out\x64>D:

D:\>cd stl\out\x64

D:\STL\out\x64>python tests\utils\stl-lit\stl-lit.py ..\..\tests\std\tests\VSO_0000000_vector_algorithms -v
stl-lit.py: D:\STL\tests\std\lit.cfg:4: fatal: You seem to be running Lit directly -- you should be running Lit through <build>/tests/utils/stl-lit/stl-lit.py, which will ensure that the right Lit configuration file is used.

Reproduce the failure using a Windows subst drive and the command shown in the issue. Start with tests/utils/stl-lit/stl-lit.py and tests/std/lit.cfg, then trace why the runner treats the substituted path as a direct Lit invocation. Done means the cited test command runs successfully from the substituted drive.