Update actions/checkout in GitHub Actions to v3

[striezel]

Updates the actions/checkout action used in the GitHub Actions workflow to its newest major version.

Changes in actions/checkout:

v3.1.0

• Use @actions/core saveState and getState
• Add github-server-url input

v3.0.2

• Add input set-safe-directory

v3.0.1

• Fixed an issue where checkout failed to run in container jobs due to the new git setting safe.directory
• Bumped various npm package versions

v3.0.0

• Update to node 16

As far as I can tell this should all be backwards compatible, so I do not expect any breakage.

Contributes to martinmoene/nonstd-lite-project#70.