ci: replace tac with awk to fix macOS runner compatibility - #575

tac is a GNU coreutils command not available on macOS runners, causing the Homebrew PR template step to fail with tac: command not found.

Change

Replaced the tac | sed | tac pattern (which marks the last unchecked checkbox in the PR template) with a portable awk one-liner that produces identical output:

# Before
TEMPLATE=$(echo "$TEMPLATE" | tac | sed '0,/- \[ \]/s/- \[ \]/- [x]/' | tac)

# After
TEMPLATE=$(echo "$TEMPLATE" | awk 'BEGIN{found=0} /- \[ \]/{lines[NR]=$0; found=NR; next} {lines[NR]=$0} END{for(i=1;i<=NR;i++){if(i==found){sub(/- \[ \]/,"- [x]",lines[i])}; print lines[i]}}')

awk buffers all lines, tracks the last matching line number, and substitutes only that occurrence on output — same semantics, no GNU dependency.