CI: fix qhelp preview
CI: fix qhelp preview

The command to gather the changed files uses NULL character terminated "lines",
therefore we should supply the `-z` flag to `basename` as well. Otherwise we
end up calling `git grep -l "\n"` which would list all files containing a newline.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql