Update `CONTRIBUTING.md` to add `--noexperimental_check_desugar_deps` to build commands

### Description

Seems like `--noexperimental_check_desugar_deps` is needed to build this repo, but this is not mentioned in the CONTRIBUTING.md

### Steps to Reproduce

If I run `build :axt_m2repository` or `test //core/javatests/... --spawn_strategy=local --host_force_python=PY2`
I run into this issue: https://github.com/bazelbuild/rules_android/issues/320

```
singlejar_cc_bin: external/remote_java_tools_linux/java_tools/src/tools/singlejar/singlejar_main.cc:27: Desugar checking not currently supported in Bazel.
```

Adding `--noexperimental_check_desugar_deps` works around it as described in https://github.com/bazelbuild/rules_android/issues/320

Update CONTRIBUTING.md, focusing on the documented `build :axt_m2repository` and `test //core/javatests/... --spawn_strategy=local --host_force_python=PY2` commands. Verify the documentation includes `--noexperimental_check_desugar_deps` as required, and confirm the commands no longer produce the reported desugar-checking error.

Written by the indexing model from the issue text.