Really only run eventlogging on --make mode

I really found a lot of complicated ways to make this not work. And a simple way to make it actually work.

Here's Cabal:

assigned to @chreekat

enabled an automatic merge when the pipeline for 969fb14a succeeds

canceled the automatic merge

https://gitlab.haskell.org/ghc/head.hackage/-/pipelines/53718 is running on this branch with EVENTLOGGING=1

Merging now after a successful local run.

merged

[b@kuusi:~/HF/clones/head.hackage]$ du -h eventlogs.tar.xz
15M     eventlogs.tar.xz

[b@kuusi:~/HF/clones/head.hackage]$ du -sh ci/run/eventlogs
73M     ci/run/eventlogs

[b@kuusi:~/HF/clones/head.hackage]$ ls ci/run/eventlogs | wc
    341     341   31622

So this will consume 15MB per nightly run, give or take.