Fix workflow parse failure from an empty expression in a comment

The nightly-probe comment used a literal \${{ }} to refer to GitHub
expression expansion. GitHub substitutes expressions in run blocks
before the shell sees them, so the empty expression made the whole
workflow unparseable ("An expression was expected") and broke every
dispatch mode, including source builds. Reworded; comment-only change,
no behavioral difference. Introduced by the probe hardening commit
50b6a8e, which therefore never ran in Actions.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: nej1gotnochill

.github/workflows/benchmark.yml

@@ -1523,7 +1523,7 @@ jobs:
           # step output. Restrict it to a shell-safe charset at capture time so
           # no shell metacharacter can ever ride along, and anchor-validate
           # before use. Consumers receive it via env:, never via textual
-          # ${{ }} expansion into a generated script.
+          # GitHub expression expansion into a generated script.
           filename=$(printf '%s' "$listing" | grep -oE 'OpenMS-[-A-Za-z0-9._+~]+-Debian-Linux-x86_64\.deb' | head -1)
           if [[ -z "$filename" ]] || ! [[ "$filename" =~ ^OpenMS-[-A-Za-z0-9._+~]+-Debian-Linux-x86_64\.deb$ ]]; then
             echo "ERROR: no x86_64 Debian .deb with the expected shell-safe filename in $base/ (see archive_listing.html in the probe artifact)" >&2