security/clamav-lts: Fix plist error

Fix plist error when user sets CLAMAVUSER to non-default value.

This is direct commit to 2021Q4 branch as the commit that causes plist
error is also direct one.

PR: 259663
Reported by: ml at netfence dot it
Fixes: 2da7ca992591 (security/clamav: Update to new bugfix release 0.103.4)
Approved by: fluffy (ports-secteam)