Allow overwriting build cmake step in workspace config

Allow syntax such as

{
  "modules": {
   ...
    "armarx/skills/control": {
      "build": {
        "cmake": {
          "env": {
            "CMAKE_BUILD_PARALLEL_LEVEL": "1"
          }
        }
      }
    },
    ...
  }

in the workspace config when the module is defined with the "build": "cmake" shortcut.

This allows, e.g., overwriting the number of cores used for specific modules as in the example above.

@christophpohl FYI