Fix base_build() with a proper support for multi-package-building through conda-build

This MR fixes build.base_build() to properly handle building multiple versions of the same package according to what is defined in conda_build_config.yaml.

conda/meta.yaml

@@ -24,7 +24,6 @@ build:
 requirements:
   host:
     - python {{ python }}
-  build:
     - pip
   run:
     - python