[infra] Optimize shards for the analyzer builders

Fixes: b/237755665
Change-Id: I11c066e8db7665ccd15ee671e2a706759cb1ef5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250386
Auto-Submit: Alexander Thomas <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: William Hesse <[email protected]>

Author: athomas

tools/bots/test_matrix.json

@@ -3272,7 +3272,7 @@
             "-nanalyzer-unittest-asserts-${mode}-${system}",
             "pkg/analyzer"
           ],
-          "shards": 2,
+          "shards": 4,
           "fileset": "analyzer_unit_tests"
         },
         {
@@ -3281,7 +3281,7 @@
             "-nanalyzer-unittest-asserts-${mode}-${system}",
             "pkg/analysis_server"
           ],
-          "shards": 2,
+          "shards": 4,
           "fileset": "analyzer_unit_tests"
         },
         {
@@ -3290,7 +3290,7 @@
             "-nanalyzer-unittest-asserts-${mode}-${system}",
             "pkg/nnbd_migration"
           ],
-          "shards": 2,
+          "shards": 1,
           "fileset": "analyzer_unit_tests"
         },
         {