Don't try overwriting mod cache during nightly build

[wlhlm]

Fixes a warning we are getting in the nightly jobs:

Run actions/cache/save@v4
/usr/bin/tar --posix -cf cache.tzst --exclude cache.tzst -P -C /home/runner/work/DreamAssemblerXXL/DreamAssemblerXXL --files-from manifest.txt --use-compress-program zstdmt
Failed to save: Unable to reserve cache with key daxxl-Linux-2024-w44, another job may be creating this cache. More details: Cache already exists. Scope: refs/heads/master, Key: daxxl-Linux-2024-w44, Version: 48840183904feb892faeaa56412d7014a19c2d726aad7885122645d312345145
Warning: Cache save failed.

The cache action doesn't support updating a cache in place, so since we are trying to use the cache for a week, all jobs after the first in the week will try to save into an existing cache repo and fail.