ci: note why the fresh clone needs no mtime sort

The mcpp side of this cross-validation had to sort by mtime — its target/
is restored from a cache and `find … | head -1` returned a binary an
earlier push had left, with the right version string and the wrong code.
Here $src is a fresh clone, so the plain form is correct; `-printf` is a
GNU extension and one of the runners reaching this line is macOS.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -117,6 +117,11 @@ jobs:
             # above, which is exactly what removing the file leaves.
             rm -f "$src/.xlings.json"
             ( cd "$src" && mcpp build --release )
+            # ⭐ `$src` is a FRESH clone each run, so `target/` holds exactly what
+            # this step just built and there is no earlier fingerprint directory
+            # to pick by mistake. `-printf` would be the safer form on a cached
+            # tree and is a GNU extension this must not use — one of the runners
+            # that reaches this line is macOS.
             built=$(find "$src/target" -type f -name mcpp -perm -u+x | head -1)
             [ -n "$built" ] || { echo "::error::mcpp did not build from $MCPP_SOURCE_REF"; exit 1; }
             echo "$(cd "$(dirname "$built")" && pwd)" >> "$GITHUB_PATH"