Reduce GCC build set.

runtime is not a proper subset of create_sdk.

Change-Id: Ieb393029481475e866daf4eb2aa3280045c17d3b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508703
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

tools/bots/test_matrix.json

@@ -1601,8 +1601,9 @@
           "arguments": [
             "--mode=debug",
             "--no-clang",
-            "runtime",
-            "runtime_precompiled"
+            "dart",
+            "dartvm",
+            "dartaotruntime"
           ]
         },
         {
@@ -1620,8 +1621,9 @@
           "arguments": [
             "--mode=release",
             "--no-clang",
-            "runtime",
-            "runtime_precompiled"
+            "dart",
+            "dartvm",
+            "dartaotruntime"
           ]
         },
         {