Temporarily disable the mockito builder for better build perf.

The mockito builder is only needed for NNBD code and the default behavior
is for it to run on every `.dart` file and fully resolve each of those
files, which is slow. **Disabling this builder should improve local and CI
build times.**

Author: evanweible-wf

build.yaml

@@ -0,0 +1,8 @@
+targets:
+  $default:
+    builders:
+      # mockito's builder is expensive and is not needed until this package is
+      # migrated to null-safety. At that point, it should be scoped only to
+      # relevant files.
+      mockito:mockBuilder:
+        enabled: false