abuild-keygen -ain hangs on user input when run non-interactively

abuild-keygen -ain will run cp -i ... to prevent overwriting the key when it already exists. When running non-interactive, this will cause abuild-keygen to hang.

This is mainly an issue in CI processes, where abuild-keygen will be executed multiple times. For example in alpine-gitlab-ci it has been worked around by patching the cp command to leave out the -i parameter, but an even easier work-around is to pipe yes into abuild-keygen -ain.

This also is an issue in the test-suite for abuild itself.

To fix this, we could add a -f flag to abuild-keygen so that existing keys are overwritten without question.