Implemented GitHub Actions Workflow for bazel build

[leozz37]

GitHub Actions

Implemented a CI to build bazil that runs on every push or pull request and a badge on README.md.

An example of it successfully running here.

[ckennelly]

Is this step necessary?

It looks like some of the virtual environments for Github Actions already install bazel (or have a helper ready to go): https://github.com/actions/virtual-environments/blob/main/images/linux/scripts/installers/bazel.sh

I'm also looking at https://github.com/googleapis/google-cloud-cpp/pull/4333/files for reference.

[AustinSchuh]

@fweikert, @philwo What about bazelci here?

[philwo]

@fweikert, @philwo What about bazelci here?

Thanks for mentioning :) Using Bazel CI would be straight-forward here, too, basically a matter of checking in this file as .bazelci/pipeline.yml:

---
tasks:
  ubuntu1804:
    build_targets:
    - "//..."
    test_targets:
    - "//..."

and pinging us here https://github.com/bazelbuild/continuous-integration/issues/new/choose so that we can create the pipeline on Bazel's CI.

Some docs about Bazel's CI: https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md