CI: reduce xz compression for non release/nightly jobs
Reduce XZ compression level for regular jobs (it is bumped to 9 for releases and nightly jobs). In my experiments I've got the following bindist size in the given time for each compression level (with the quick flavour):
XZ_OPT Time Size
-9 4m06s 112 MB
-8 4m00s 114 MB
-7 3m50s 116 MB
-6 (default) 3m40s 118 MB
-5 2m47s 123 MB
-4 1m57s 134 MB
-3 1m03s 129 MB
-2 49.73s 136 MB
-1 37.72s 142 MB
-0 34.40s 156 MB