79e2b6d specfile: Use Makefile install target

Authored and Committed by kfan 3 years ago
    specfile: Use Makefile install target
    
    To prepare for adding kola external tests, the Makefile now has
    the install target
    (https://github.com/coreos/console-login-helper-messages/pull/56).
    Let specfile use `make install` to avoid having multiple sources
    of truth.
    
    Other changes in upstream repo:
    https://github.com/coreos/console-login-helper-messages/pull/57
    https://github.com/coreos/console-login-helper-messages/pull/63