fix(ci): stop the layer-cache prune from being able to fail a build (#7252)

The settle loop added in 4f65103 reads `cur="$(total)"`, and composite steps
run under `bash -e -o pipefail`, where an assignment takes its command
substitution's exit status. So any failing `buildctl du` aborts the step and
fails the build. `echo "$(total)"` survives the same failure, which is why this
was invisible in the paths that ran first.

That is not hypothetical. Deleting a sticky disk out from under a running job
makes buildkitd panic inside cache.(*cacheManager).DiskUsage, and it took a
Build AMD64 job down that way. `grep` also exits 1 whenever du prints no Total
line, so an empty cache would have done it too.

A cache-hygiene step must never be able to fail a deploy, so `total()` now always
returns 0. The prune's own failure was already handled — it is an `if` condition,
and a failing condition does not trip `-e`.

Author: waleedlatif1

.github/actions/docker-build/action.yml

@@ -119,7 +119,16 @@ runs:
 
         # Print the whole Total line rather than picking a column: buildctl's du
         # table is whitespace-aligned and its layout is not a stable contract.
-        total() { sudo buildctl --addr "$addr" du 2>/dev/null | grep -iE '^total:' | tr -s ' \t' ' '; }
+        #
+        # The trailing `|| true` is load-bearing. Composite steps run under
+        # `bash -e -o pipefail`, where `cur="$(total)"` takes the substitution's
+        # exit status, so a failing du would abort the step and fail the build --
+        # `echo "$(total)"` survives but the assignment in the settle loop does
+        # not. buildctl exiting non-zero here is entirely plausible: deleting a
+        # sticky disk out from under a running job makes buildkitd panic inside
+        # DiskUsage, and grep also exits 1 whenever the table has no Total line.
+        # Cache hygiene must never be able to fail a deploy.
+        total() { sudo buildctl --addr "$addr" du 2>/dev/null | grep -iE '^total:' | tr -s ' \t' ' ' || true; }
         echo "before prune -> $(total)"
 
         if sudo buildctl --addr "$addr" prune --all --keep-storage "$KEEP_MB"; then