Automatic merge from submit-queue

bazel: add a config setting to control embedding kubernetes-src.tar.gz

**What this PR does / why we need it**: currently a change anywhere in the tree will cause `kubernetes-src.tar.gz` to need to be regenerated, and thus also the server and node tarballs. All of these operations are slow, so for the sake of developer productivity, only include `kubernetes-src.tar.gz` when we need it (e.g. if we were doing a real release).

I don't have metrics on how much of an effect this has, but I expect it should help incremental builds, especially those that don't affect any node/server targets.

To embed the srcs tarball with this change, you'd run
```console
bazel build //build/release-tars --define EMBED_LICENSE_TARGETS=true
```

**Release note**:

```release-note
NONE
```