fix: Switch to actions/checkout@v2

[cwhitten]

attempting to address:

git checkout --progress --force b679ac13b2c48def229f873545fa1258bbf0decd 
359##[error]fatal: reference is not a tree: b679ac13b2c48def229f873545fa1258bbf0decd 
360Removed matchers: 'checkout-git' 
361##[error]Git checkout failed with exit code: 128 
362##[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"'.

fixes #2221

[github-actions]

Coverage remained the same at 40.785% when pulling 48e2e22 on cwhitten/gh-actions-v2 into ceb8186 on master.