test: update keys/Makefile to clean and build all

Currently when running make clean, followed by make not all certificates
get genenerated. In addition there are also the following error messages
related to the startdate in agent8-cert.pem, and agent9-cert.pem:
start date is invalid, it should be YYMMDDHHMMSSZ

After this commit it is possible to perform the following commands:
$ make clean
$ make
$ make test

PR-URL: https://github.com/nodejs/node/pull/19975


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>