This commit changes the way the action determines if running in action
or runner mode. There is now an environment variable that is set at the
beginning of the process and elsewhere in the process, we can check to
see if the variable is set.
As we move towards analysing the merge commit for pull requests by
default, we should stop sending `/refs/pull/n/head` rather than
`refs/pull/n/merge` _unless_ the checked-out SHA has actually changed.
Here we assume that any change (compared to GITHUB_SHA) indicates that
`git checkout HEAD^2` has been run earlier. This may sometimes be
incorrect (e.g. `git checkout mybranch`), but in that case the ref
would be wrong either way.
