Enable ASan for github CI.

Untested (because I don't know how to test such changes without merging
them), but seems plausible based on the documentation at
https://docs.github.com/en/actions/learn-github-actions/variables#about-environment-variables

.github/workflows/toybox.yml

@@ -26,6 +26,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - env:
+      ASAN: 1
     - name: Setup
       run: sudo apt-get install build-essential
     - name: Configure