Allow overriding CUSTOM_COMPILE_COMMAND (#472)

This way if users have bazel wrappers the error message can be correct

python/pip_install/pip_compile.py

diff --git a/python/pip_install/pip_compile.py b/python/pip_install/pip_compile.py
index d5fbd83..1c22d2c 100644
--- a/python/pip_install/pip_compile.py
+++ b/python/pip_install/pip_compile.py
@@ -54,7 +54,7 @@
 # $(rootpath) in the workspace root gives ./requirements.in
 if update_target_pkg == ".":
     update_target_pkg = ""
-update_command = "bazel run //%s:%s" % (update_target_pkg, update_target_name)
+update_command = os.getenv("CUSTOM_COMPILE_COMMAND") or "bazel run //%s:%s" % (update_target_pkg, update_target_name)
 
 os.environ["CUSTOM_COMPILE_COMMAND"] = update_command