some github action tweaks for slow actions

Hi folks,

I really like the recent performance/JMH GitHub action improvements, but
they cause a lot of CI activity when in some cases it isn't really needed.
I have modified them so that if the last commit message in any push/PR
contains "GROOVY-" they will fire, so they should fire alot of the time,
but for something like "minor refactor: fix typo", they will be skipped.

I have just done this on master but if it seems useful we can push to other
branches. I went with these actions for now:

groovy-build-performance
groovy-jmh
groovy-jmh-classic
groovy-build-coverage

But we can tweak which ones if these prove to be the wrong list.

Thanks, Paul.