[ci] Bump number of shards for test_build to 5

I noticed `test_build` can take a while so let's bump the number of shards to 5.

Author: poteto

.github/workflows/runtime_build_and_test.yml

@@ -373,9 +373,11 @@ jobs:
           # TODO: Test more persistent configurations?
         ]
         shard:
-          - 1/3
-          - 2/3
-          - 3/3
+          - 1/5
+          - 2/5
+          - 3/5
+          - 4/5
+          - 5/5
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4