MINIFICPP-1543 Add 'brew update' to MacOS CI jobs
MINIFICPP-1543 Add 'brew update' to MacOS CI jobs

... and remove packages which are already installed, because these cause
`brew install` to return with a non-zero status, for some reason.

Signed-off-by: Arpad Boda <aboda@apache.org>

This closes #1051

dtrodrigues/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp