(core) add CI github action for grist-core

Summary:
Activate CI for grist-core using github actions.  Can be improved
with caching but starting simple.

Test Plan: tested in a fork of grist-core

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2771

buildtools/prepare_python.sh

@@ -1,8 +1,11 @@
 #!/bin/bash
 
+set -e
+
 if [ ! -e venv ]; then
   virtualenv -ppython2.7 venv
 fi
 
 . venv/bin/activate
+
 pip install --no-deps -r sandbox/requirements.txt