added pre_build task stub

[chenkasirer]

@Licini how do you feel about adding a pre-build hook task here that will be triggered in https://github.com/compas-dev/compas-actions.build running invoke test and invoke lint.

The reason this would be useful is that now with compas_pb we need to generate the protobuf files before the tests can be run, since these are imported in the library.

so to run the tests, we need:

1. run the task that compiles the protobuf messages
2. invoke test

so currently I have to override the compas build action and do it manually.

if we have this "hook", any package that need to do stuff before running the test can implement it..

dunno if there's maybe a native solution in github actions or otherwise.

[Licini]

@chenkasirer sure we can add a flag to run prebuild option. What would be the command to trigger prebuild? invoke prebuild?