Contribution description

The no-merge-keyword expressions were prone to false positives, because in #21803 I removed the ^ of the regex, that tells it to only match from the line beginning.

Testing procedure

Current master matches to e update and therefore causes a false positive.

cbuec@W11nMate:~/RIOTstuff/riot-guides/RIOT$ echo "cpu/stm32: style update of rtc_all.c" | grep -i -f dist/tools/pr_check/no_merge_keywords
cpu/stm32: style update of rtc_all.c

This PR does not.

cbuec@W11nMate:~/RIOTstuff/riot-guides/RIOT$ echo "cpu/stm32: style update of rtc_all.c" | grep -i -f dist/tools/pr_check/no_merge_keywords

Issues/PRs references

Fixes regression introduced in #21803.