ci: enable IPO/LTO for nightly builds and disable for pre-merge
Let's SKIP LTO in pre-merge (currently only debian-build-testing
and Fedora build).
Let's DO LTO in nightly scheduled runs for every build (incl. these distributed to the runners).
Deprecated: !20375
ci: enable IPO/LTO for nightly builds
1. nightly takes a long, the extra few minutes for linking don't matter
2. nightly will run faster, since where it's CPU dependent it's at least +5% perf
3. it may reveal some painful areas of common code or driver.