Add a workaround for fstrim command not working in some FS

juadderequested to merge
Kppqju77/arch-boxes:fstrim into master

Today build is failing when fstrim is not supported on the FS. This workaround enables the build to continue even when trimming is failing, by having a workaround for set -o errexit option at the beginning. We could also unset this property.

Please advise if any adjustment should be done.

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