Shell quote environment variable values when dumping them in dump_env.py

The mozconfig output parsing code already (mostly) handles shell quoted strings, because that's what `set` outputs. By quoting environment variable values, we avoid a bunch of problems with "weird" values.

python/mozbuild/mozbuild/action/dump_env.py

@@ -6,5 +6,11 @@
 # native paths printed on Windows so that these paths can be incorporated
 # into Python configure's environment.
 import os
+import sys
+
+sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
+
+from shellutil import quote
+
 for key, value in os.environ.items():
-    print('%s=%s' % (key, value))
+    print('%s=%s' % (key, quote(value)))