Disable MSYS/MSYS2 path conversion, which interferes with Bazel.

Change-Id: I0a0966667baf39310eba885078b99200612e014e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62790
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>