[bazel] Add BazelBuild job for //example/external_client:use_ganesh_gl

Bug: b/256860862
Change-Id: I25b02eb0c3997b3e6e116a29d39cd59931a90b82
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/896461
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Eric Boren <borenet@google.com>
Commit-Queue: Eric Boren <borenet@google.com>