add custom endpoint environment variable

roles/matrix-synapse/templates/goofys/env-goofys.j2

@@ -1,2 +1,3 @@
 AWS_ACCESS_KEY={{ matrix_s3_media_store_aws_access_key }}
-AWS_SECRET_KEY={{ matrix_s3_media_store_aws_secret_key }}
+AWS_SECRET_KEY={{ matrix_s3_media_store_aws_secret_key }}
+{% matrix_s3_media_store_custom_endpoint_enabled %}ENDPOINT={{ matrix_s3_media_store_custom_endpoint }}{% endif %}