fix: ensure dosdevices subdir exists in prefix

[R1kaB3rN]

The proton script can raise a FileNotFoundError in its prefix upgrade due to the dosdevices subdirectory not existing. Discovered this bug when looking into a broken game with GE-Proton9-27 as the compatibility tool on Steam.

Traceback:

 Proton: Upgrading prefix from None to GE-Proton9-27 (/home/deck/.local/share/Steam/steamapps/compatdata/2697560/)
 Traceback (most recent call last):
   File "/home/deck/.local/share/Steam/compatibilitytools.d/GE-Proton9-27/proton", line 1924, in <module>
     g_session.init_session(sys.argv[1] != "runinprefix")
   File "/home/deck/.local/share/Steam/compatibilitytools.d/GE-Proton9-27/proton", line 1816, in init_session
     g_compatdata.setup_prefix()
   File "/home/deck/.local/share/Steam/compatibilitytools.d/GE-Proton9-27/proton", line 970, in setup_prefix
     os.symlink("../drive_c", self.prefix_dir + "/dosdevices/c:")
 FileNotFoundError: [Errno 2] No such file or directory: '../drive_c' -> '/home/deck/.local/share/Steam/steamapps/compatdata/2697560/pfx//dosdevices/c:'

Link to upstream report: ValveSoftware#8213