OSSL_STORE: Avoid testing with URIs on the mingw command line

Some URIs get "mistreated" (converted) by the MSYS run-time.
Unfortunately, avoiding this conversion doesn't help either.

    http://www.mingw.org/wiki/Posix_path_conversion



Fixes #4314

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4322)