Fix 'show' displaying UNC paths on windows

[brson]

After #1172 several test cases in cli-rustup will be ignored because rustup is unexpectedly showing UNC (\\?\) file paths (and show_toolchain_override is disabled today), probably because of the normalization being done to the paths during the override directory walk.

When these paths are reported, they probably need to be converted back to non-UNC paths where it is unambiguous, or perhaps should not be converted to UNC in the first place. I do not know the semantics here well enough to know why the paths need to be converted to UNC in the first place, or in which cases they don't need to be. Maybe @Diggsey or @retep998 can say.

[retep998]

It's not that we're normalizing paths, it's that we're canonicalizing paths, which results in getting root local device paths back. If all we're doing is printing out paths for the user, then stripping the initial \\?\ prefix is probably just fine.

[tbu-]

That doesn't work for paths containing special characters or reserved words.