os.Getwd: wrap error on windows and plan9

The errors from os are supposed to be wrapped to add some context,
but in this particular case a raw syscall error is returned.

Change-Id: I1b98dbd7b385c5c1cea79a1b0ec7201ca9bdca40
Reviewed-on: https://go-review.googlesource.com/c/go/+/606657


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