[Fix] (CI): narrow the trace fixture Git LFS fallback to the files mirrors lost

The fetch script tries git.hit.moe, then the repo.miawa.cn mirror, and only
then Git LFS, but it bailed out of the mirror loop on the first file no
mirror could serve and then pulled the whole case from GitHub. A case whose
mirrors served every file but one paid GitHub's LFS bandwidth for all of
them.

Collect the files that survived every mirror and every retry instead, and
scope the LFS fallback to just those, matching what the local macOS retrace
helper already does.

.github/scripts/fetch-trace-fixture-lfs.sh

@@ -201,6 +201,11 @@ fetch_file_from_mirror() {
   return 1
 }
 
+# Files no mirror could serve, even after retrying every mirror. Only these fall
+# back to Git LFS, so a mirror that served the rest of the case still spares
+# GitHub the bandwidth for those files.
+mirror_failures=()
+
 fetch_from_mirror() {
   mkdir -p "${fixture_dir}"
   for file in "${files[@]}"; do
@@ -219,17 +224,19 @@ fetch_from_mirror() {
       echo "Mirror did not serve ${name}; trying the next mirror" >&2
     done
     if [ "${fetched}" -ne 1 ]; then
-      return 1
+      mirror_failures+=("${file}")
     fi
   done
+  [ "${#mirror_failures[@]}" -eq 0 ]
 }
 
 if fetch_from_mirror; then
   echo "Fetched trace fixture files for ${case_name} from mirror: ${include}"
 else
-  echo "All mirrors failed for ${case_name}; falling back to Git LFS: ${include}"
+  fallback_include="$(IFS=,; echo "${mirror_failures[*]}")"
+  echo "All mirrors failed for ${#mirror_failures[@]} of ${#files[@]} file(s) of ${case_name}; falling back to Git LFS: ${fallback_include}"
   git lfs install --local
-  git lfs pull --include="${include}" --exclude=""
+  git lfs pull --include="${fallback_include}" --exclude=""
 fi
 
 for file in "${files[@]}"; do