add a key to ctx.action dict to prevent protoc losing the default env

protobuf.bzl

diff --git a/protobuf.bzl b/protobuf.bzl
index 3998e77..14e6789 100644
--- a/protobuf.bzl
+++ b/protobuf.bzl
@@ -90,6 +90,7 @@
         arguments=args + import_flags + [s.path for s in srcs],
         executable=ctx.executable.protoc,
         mnemonic="ProtoCompile",
+        use_default_shell_env=True,
     )
 
   return struct(
@@ -141,7 +142,7 @@
     compiler.
   plugin_language: the language of the generated sources
   plugin_options: a list of options to be passed to the plugin
-  gen_cc: generates C++ sources in addition to the ones from the plugin. 
+  gen_cc: generates C++ sources in addition to the ones from the plugin.
   gen_py: generates Python sources in addition to the ones from the plugin.
   outs: a list of labels of the expected outputs from the protocol compiler.
 """