Update github workflow to checkout v2.

[xStrom]

The github actions script we have uses checkout v1. Apparently that's broken. I ran into the issue in #712 where the tests timed out and I tried to manually re-run the jobs. However manual re-running of jobs doesn't work.

git checkout --progress --force 1b3971da947c8f67e5dcb2bbc47bd206deaae3c6
##[error]fatal: reference is not a tree: 1b3971da947c8f67e5dcb2bbc47bd206deaae3c6
Removed matchers: 'checkout-git'
##[error]Git checkout failed with exit code: 128
##[error]Exit code 1 returned from process: file name '/home/runner/runners/2.165.2/bin/Runner.PluginHost', arguments 'action "GitHub.Runner.Plugins.Repository.v1_0.CheckoutTask, Runner.Plugins"'.

This issue is documented in checkout#23. The issue was closed with a claim that checkout v2 fixes it.

This PR here updates our scripts to use checkout v2.

[cmyr]

Thanks!