add support to inject other pip index (#103)

Author: simon-d-bmw

MODULE.bazel

@@ -48,6 +48,8 @@ use_repo(python)
 ###############################################################################
 pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
 pip.parse(
+    envsubst = ["PIP_INDEX_URL"],
+    extra_pip_args = ["--index-url=${PIP_INDEX_URL:-https://pypi.org/simple/}"],
     hub_name = "pip_process",
     python_version = PYTHON_VERSION,
     requirements_lock = "//src:requirements.txt",