fix dict keys usage outside of iteration

A few scenarios not caught by the linter where generators don't work
under Python 3.

BUG=chromium:997354
TEST=unittests still pass

Change-Id: Ib1f9685476b042d786b8d59863a4a8f7f015a487
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1804140
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 9546d18..fe67a0a 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -752,8 +752,8 @@
   # Make sure we display the default toolchain first.
   # Note: Do not use logging here as this is meant to be used by other tools.
   print(','.join(
-      toolchain.FilterToolchains(toolchains, 'default', True).keys() +
-      toolchain.FilterToolchains(toolchains, 'default', False).keys()))
+      list(toolchain.FilterToolchains(toolchains, 'default', True)) +
+      list(toolchain.FilterToolchains(toolchains, 'default', False))))
 
 
 def GeneratePathWrapper(root, wrappath, path):