Remove duplicate code from presubmit config (#224)

Author: fweikert

.bazelci/presubmit.yml

@@ -4,10 +4,7 @@
 # TODO(#144): When Bazel 0.29 is released, the docgen workflow will work, and
 # we can go back to using `...` for build_targets.
 ---
-platforms:
-  ubuntu1604:
-    # I don't trust my knowledge of YAML enough to factor this list into an
-    # anchor.
+all_targets: &all_targets
     build_targets:
     - "//examples/..."
     - "//experimental/..."
@@ -16,21 +13,10 @@ platforms:
     - "//tools/..."
     test_targets:
     - "..."
+platforms:
+  ubuntu1604:
+    <<: *all_targets
   ubuntu1804:
-    build_targets:
-    - "//examples/..."
-    - "//experimental/..."
-    - "//python/..."
-    - "//rules_python/..."
-    - "//tools/..."
-    test_targets:
-    - "..."
+    <<: *all_targets
   macos:
-    build_targets:
-    - "//examples/..."
-    - "//experimental/..."
-    - "//python/..."
-    - "//rules_python/..."
-    - "//tools/..."
-    test_targets:
-    - "..."
+    <<: *all_targets