tools: fix release script on macOS 10.12

Previously, we were relying on the output of gpg from git tag -v to
verify that the key selected by the releaser is the key that was used
to sign the tag. This output can change depending on the version of git
being used. Now, we just check that the output of git tag -v contains
the key selected.

Fixes: https://github.com/nodejs/node/issues/8822
PR-URL: https://github.com/nodejs/node/pull/8824


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>