cmake: do not print warning in each cmake run

cmake: do not print warning in each cmake run

cmake prints the warning of

> run-promtool-unittests is skipped due to missing promtool

everytime if the test is skipped per the test. but we only need to
get this message the first time cmake is executed. more importantly,
the value of `PROMTOOL_EXECUTABLE` is cached. so the new error message
does not provide updated information, so just stop doing so if the
cached variable is found.

Signed-off-by: Kefu Chai <kchai@redhat.com>