windows.GetFinalPathNameByHandle: Support volumes mounted as paths

[squeek502]

A volume can be mounted as a NTFS path, e.g. as C:\Mnt\Foo. In that case, IOCTL_MOUNTMGR_QUERY_POINTS gives us a mount point with a symlink value something like \??\Volume{383da0b0-717f-41b6-8c36-00500992b58d}. In order to get the C:\Mnt\Foo path, we can query the mountmgr again using IOCTL_MOUNTMGR_QUERY_DOS_VOLUME_PATH.

Fixes #19731

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Zig installed to a VHD mounted as a path:

Before:

> zig test test.zig
error: unable to find zig self exe path: FileNotFound

After:

> zig test test.zig
All 1 tests passed.

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Note: It's also possible for a volume to be mounted to multiple paths. This PR will currently always just return the first one--unsure how something like GetVolumePathNameW compares, need to test that.