setup: Make environment variables optional

To make it easier to use in tests.

setup.py

@@ -52,9 +52,9 @@ if in_source_package:
         frida_version = version_line[9:]
     long_description = None
 else:
-    frida_version = os.environ['FRIDA_VERSION']
+    frida_version = os.environ.get('FRIDA_VERSION', None)
     long_description = codecs.open(os.path.join(package_dir, "README.md"), "r", 'utf-8').read()
-    frida_extension = os.environ['FRIDA_EXTENSION']
+    frida_extension = os.environ.get('FRIDA_EXTENSION', None)
 
 index_url_pip_configs = ("global.index-url", "global.extra-index-url")