Update reference repos not gc'ing on AIX

[pshipton]

The following seems to happen on all machines, not sure if it's a real problem though.

https://openj9-jenkins.osuosl.org/view/Infrastructure/job/Update-Reference-Repos-AIX/1

16:00:59  + git gc --aggressive --prune=all
16:00:59  fatal: gc is already running on machine 'p8-java1-ibm10' pid 25428318 (use --force if not)

@AdamBrousseau