games/veloren-weekly: simplify 6054513a40b1

• Other date(1) consumers in ports/ use -u POSIX flag
• "date -u" uses UTC0 aka UTC+0 i.e., UTC with explicit offset
• Reduces line length by 6 characters

(cherry picked from commit 6d78299a54d87c2b959c760c1b4ac0a3022c9aa4)