Build Chain: Allow to specify project environment variable in pull request config

For now we have the `BUILD_MVN_OPTS` which is global to all Maven commands.

We could have some project specific Maven env var, like `OPTAPLANNER_BUILD_MVN_OPTS` or `OPTAPLANNER_CURRENT_BUILD_MVN_OPTS`

This would help to fine-tune the checks for specific use cases (for example optaplanner-quickstarts needing a `full` build of optaplanner)