packaging: Ensure we're propagating BUILD_VERSION (#1697)

BUILD_VERSION gets set by upstream scripts and should be available
whenever we're attempting to build audio in conda environments.

This solves the issue where conda packages appeared to be building a
development version instead of the actual version we were targeting
Signed-off-by: Eli Uriegas <eliuriegas@fb.com>

packaging/torchaudio/meta.yaml

@@ -27,6 +27,8 @@ requirements:
 
 build:
   string: py{{py}}
+  script_env:
+    - BUILD_VERSION
 
 test:
   imports: