build-aux: Add cleanup to gpg-authcode-sign.sh

* tools/gpg-authcode-sign.sh (cleanup): New.

When using osslsigncode it does not delete the
output file on error. Errors or cancels there
can happen easily with either timestamp problems
or a wrong password.
Additionally, if an output file exists, osslsigncode
does not write a good error message but shows
some exception.