build: reduce shard_count of allccl_test to 4

This package only has 4 tests.

Release note: none
Epic: none

Author: rickystewart

pkg/ccl/workloadccl/allccl/BUILD.bazel

@@ -46,7 +46,7 @@ go_test(
         "main_test.go",
     ],
     embed = [":allccl"],
-    shard_count = 16,
+    shard_count = 4,
     deps = [
         "//pkg/base",
         "//pkg/ccl",