ci: fix jq broken pipe in cache-prune (#38734)

Fix minor issue: `cache-prune` logs `jq: error: writing output failed:
Broken pipe` when it has nothing to delete, because the loop stops
reading as soon as it is under the limit while `jq` still has output
pending. Reading from a here-string removes the pipe.

---------

Signed-off-by: silverwind <me@silverwind.io>

.github/workflows/cache-prune.yml

@@ -30,9 +30,8 @@ jobs:
           caches=$(gh cache list --limit 1000 --sort last_accessed_at --order asc --json id,key,sizeInBytes)
           size=$(jq '[.[].sizeInBytes] | add // 0' <<< "$caches")
           echo "cache usage: $((size / 1000000)) MB"
-          jq -r '.[] | "\(.id) \(.sizeInBytes) \(.key)"' <<< "$caches" |
-            while [ "$size" -gt 6500000000 ] && read -r id bytes key; do
-              echo "deleting $key"
-              gh cache delete "$id"
-              size=$((size - bytes))
-            done
+          while [ "$size" -gt 6500000000 ] && read -r id bytes key; do
+            echo "deleting $key"
+            gh cache delete "$id"
+            size=$((size - bytes))
+          done <<< "$(jq -r '.[] | "\(.id) \(.sizeInBytes) \(.key)"' <<< "$caches")"