Umbra failure on macos-12 Github runners

[derekbruening]

Github deprecated the macos-11 runners (xref DynamoRIO/dynamorio#6864) but when we try to update to macos-12 umbra fails:

ASSERT FAILURE (thread 40924): /Users/runner/work/drmemory/drmemory/umbra/umbra_64.c:518: seg->shadow_end[seg_map_idx] > seg->shadow_base[seg_map_idx] (wrong shadow segment range)

We may have to disable Mac from the package builds until this is fixed.

[derekbruening]

PR #2509 is where this is observed.