tests: Put a workaround for semihosted environment.

* tests/openpgp/defs.scm [*win32*]: Use --build-prefix option.

On the semihosted environment, output of simple gpgconf
--list-components includes drive name (like Z:), which results failure
of command invocation. This is a workaround.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>