ci: check that no run: block interpolates an expression

A ${{ }} lands in the script text before bash reads the line, and
actionlint only flags the contexts it already knows are attacker
controlled. Nothing enforced the rule the workflow follows. Also drops the
matrix table lookup, which has no table to read now.

Claude-Session: https://claude.ai/code/session_01ShuAy8q8ZfPi8KHxwc8JpQ

.github/scripts/workflow-refs.sh

@@ -1,17 +1,17 @@
 #!/usr/bin/env bash
-# Checks that everything the workflows name actually exists: composite actions,
-# make targets, and the scripts a run: block invokes.
+# Checks that everything the workflow names actually exists: composite actions,
+# make targets, and the scripts a run: block invokes. Then checks that no run:
+# block interpolates a `${{ }}`.
 #
 # This is the class actionlint does not cover. `uses: ./.github/actions/typo`
-# lints clean and fails only when the job runs, and these workflows are
-# dispatch-only or push-triggered, so that first run is after merge.
+# lints clean and fails only when the job runs, and the folio jobs and the
+# release job never run on a pull request, so that first run is after merge.
 set -euo pipefail
 
 root="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
 
 ROOT="$root" python3 - <<'PY'
 import glob
-import json
 import os
 import re
 import sys
@@ -75,14 +75,6 @@ for path in sorted(glob.glob(os.path.join(root, ".github/workflows/*.yml"))):
     commands = re.sub(r"#[^\n]*", "", body)
     for name in re.findall(r"\bmake\s+([a-z][a-z0-9-]*)\b", commands):
         wanted.add(name)
-    # `make "sanderling-$SANDERLING"` is resolved from the matrix that feeds it
-    if "sanderling-$SANDERLING" in body:
-        table = re.search(r"examples='(\[.*?\])'", body, re.S)
-        if table is None:
-            sys.exit("workflow-refs: %s builds a make target from $SANDERLING but its "
-                     "examples table could not be read" % rel(path))
-        for entry in json.loads(table.group(1)):
-            wanted.add("sanderling-%s" % entry["sanderling"])
 for name in sorted(wanted):
     report(name in targets, "make %s" % name)
 
@@ -97,6 +89,51 @@ for name in sorted(scripts):
     if os.path.isfile(full):
         report(os.access(full, os.X_OK), "%s is executable" % name)
 
+# --- expressions in a run: block ---------------------------------------------
+# A `${{ }}` is substituted into the script text before bash reads the line, so
+# an expression carrying text someone else wrote runs as a command. Values reach
+# a run: block through env instead. actionlint flags only the contexts it knows
+# are attacker-controlled, and a matrix value or a dispatch input is not on that
+# list.
+print("\nrun: blocks free of ${{ }}:")
+
+
+def run_blocks(text):
+    lines = text.split("\n")
+    i = 0
+    while i < len(lines):
+        head = re.match(r"^(\s*(?:-\s+)?)run:(.*)$", lines[i])
+        if head is None:
+            i += 1
+            continue
+        column, rest = len(head.group(1)), head.group(2).strip()
+        start, body = i + 1, []
+        if rest in ("|", "|-", "|+", ">", ">-", ">+", ""):
+            i += 1
+            while i < len(lines) and (not lines[i].strip()
+                                      or len(lines[i]) - len(lines[i].lstrip()) > column):
+                body.append(lines[i])
+                i += 1
+        else:
+            body.append(rest)
+            i += 1
+        yield start, "\n".join(body)
+
+
+blocks = 0
+for path in workflow_files():
+    hits = []
+    for line, body in run_blocks(open(path).read()):
+        blocks += 1
+        hits += ["line %d: %s" % (line, hit) for hit in re.findall(r"\$\{\{.*?\}\}", body, re.S)]
+    report(not hits, rel(path), "  (%s)" % ("; ".join(hits) if hits else "clean"))
+
+# Same reason as the local action count above: a scanner that reads no run:
+# block at all would pass every file it never looked at.
+if blocks == 0:
+    sys.exit("workflow-refs: found no run: block at all, so this check is not "
+             "reading the workflows it claims to read")
+
 print("\n%d references checked" % checked)
 if problems:
     sys.exit("workflow-refs: unresolved: %s" % ", ".join(problems))