Spawn: prune non-{C:,Z:} drive letters from prefix at launch

Wine canonicalises paths against the dosdevices map and prefers the most
specific (longest-prefix) drive letter.  An external symlink such as
dosdevices/x: -> /home/<user> turns every Z:\home\<user>\... binary path
into X:\..., which confuses tools and obscures launch logs.

PrefixSetupRunner::stepDriveCleanup already enforces the {C:, Z:} policy
but only runs as part of the explicit prefix-setup pipeline; drives
added later (Faugus, manual edits, modlist installers) survive forever.
Mirror that cleanup at launch in spawn.cpp's pruneExtraDrives() so every
Proton launch sees a known-good dosdevices layout.

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