Update the Ubuntu containers for the commit and formatting checks to
Ubuntu 20.04.
The latest version of the checkout action requires git v2.18 in order to
checkout the code as a git repo, which is required for these workflows.
With older versions of git the action will download the code as a
tarball and lack the .git directory.
All builds have been using the same cache id for ~/.cargo which
could lead us to conflict situations which is what I think we are
seeing with the commit-check job.
On a pull request, attempt to compile all commits from
the base branch to the head of the PR branch.
The job is in a separate workflow file to limit it to
pull-requests only, as the base branch is not available
on push (something to look into).