Installing a build key requires root permissions, something that is not
possible in all cases, most notably on the builders.
Instead of generating a key each time, which results in many keys being
generated each test suite run, check whether a key is already present by
using `abuild-sign --installed`, and only generate a key when that
returns false.
Remove the SUDO environment variable, as it depends on what's installed
and configured on the host what implementation should be used.
It would not suffice to add sudo or doas as a makedepend, as it would
also need to be configured to allow abuild-keygen -ain to be executed
without password.
