Harden find_replace.sh CI helper against arg splitting

The find/replace helper piped `grep -rl` output into `xargs gsed` using
xargs' default whitespace splitting and quote processing, and passed the
search pattern to grep/gsed without terminating option parsing. Paths
containing spaces (or values beginning with `-`) could be split into
multiple arguments or interpreted as options rather than data.

Use NUL-delimited matching (`grep -rlZ | xargs -0`) so filenames are
passed verbatim, and add `--` after grep's options to end option parsing.
Behavior is otherwise unchanged.

.github/scripts/find_replace.sh

@@ -3,4 +3,4 @@ old_text=$1
 new_text=$2
 echo "Old text: ${old_text}"
 echo "New text: ${new_text}"
-grep -rl "${old_text}" . | xargs gsed -i -e '1h;2,$H;$!d;g' -e "s/${old_text}/${new_text}/g"
+grep -rlZ -- "${old_text}" . | xargs -0 gsed -i -e '1h;2,$H;$!d;g' -e "s/${old_text}/${new_text}/g"