[ci] At least one presubmit task should attempt to build everything (#1407)

We've had several cases where we've needed to land additional PRs to fix
things once they'd landed because we weren't building the entire repo.

We don't need to do this for our entire matrix of versions and OSs, but
doing this in one place would be Really Nice.

Author: shs96c

.bazelci/presubmit.yml

@@ -6,6 +6,8 @@ tasks:
       # https://github.com/bazelbuild/rules_jvm_external/pull/316
       COURSIER_CACHE: /tmp/custom_coursier_cache
       REPIN: 1
+    build_targets:
+      - "//..."
     shell_commands:
       - bazel run @regression_testing_coursier//:pin
       - bazel run @regression_testing_gradle//:pin