Update default CodeQL version to codeql-bundle-20201008

[rneatherway]

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.

[robertbrignull]

Release LGTM. Obviously all the tests pass, I've checked the logs and queries are not being compiled from scratch, and I've downloaded the artefact and given it a look too.

I am surprised that the codeql-runner artefacts have changed size. Down to 45MB from 70MB. I'm not sure why this is and they're still 70MB if I build them on my machine. The ones I download from the release still seem to work though.

[robertbrignull]

I added a workflow to build and upload the runner artefacts: https://github.com/github/codeql-action/runs/1246905697?check_suite_focus=true

So they are 45MB when built on actions. Our hypothesis is that it's the node version. I have v13 on my machine here, whereas @rneatherway and actions are on v12, so this could be the reason. Regardless, I don't think this needs to hold up this release.