chore(ci): enable s3-cache on all runner profiles

Adds s3-cache extra to all runner profiles in runs-on.yml. This
replaces GHA cache with runs-on's S3-backed cache, which should be
faster since it's colocated with the runners.

Note: takes effect after merge to main (public repo limitation).

Co-authored-by: Claude <noreply@anthropic.com>

Author: samantha-earthmover

.github/runs-on.yml

@@ -2,13 +2,17 @@ runners:
   small-ubuntu24:
     cpu: [2, 4]
     image: ubuntu24-full-x64
+    extras: [s3-cache]
   default-ubuntu24:
     cpu: [4, 8]
     image: ubuntu24-full-x64
+    extras: [s3-cache]
   large-ubuntu24:
     cpu: [8, 16]
     image: ubuntu24-full-x64
+    extras: [s3-cache]
   arm-ubuntu24:
     cpu: [4, 8]
     family: ["c7g", "m7g", "r7g"]
     image: ubuntu24-full-arm64
+    extras: [s3-cache]