Actions: opt to run with either cfdm release or master

There are times when in context we want to run the cf-python test suite (run-test-suite.yml workflow) under Actions CI where the cfdm dependency has a certain version, and other times when to test appropriately we need to run against the current state of the cfdm master branch.

For the former case, usually the most suitable version would be the latest-release cfdm, but perhaps the ability to run with an arbitrary version could be useful, e.g. to check for compatible versions (perhaps under a job matrix in a new compatibility workflow, or similar).

Therefore it would be good to enable some way to specify which of these cfdm states a PR should run off in the run-test-suite.yml workflow. Ideally we can make that one workflow specification configurable by some trigger to run off another version to the default (see below), rather than having a separate workflow.

In fact, I think in most cases we want to run with the development cfdm, rather than a release one, as quite often we commit changes that would break the end-to-end system if cf and cfdm are not both updated appropriately. So as a starting point, I will change the workflow to run off the master state of cfdm, as currently we run with the latest version available on the conda channels (and that is not even up-to-date so as to be the latest release).

Start with .github/workflows/run-test-suite.yml and inspect how the workflow currently obtains cfdm from the conda channels. Define how a run selects the cfdm release or master state, including whether an arbitrary version or matrix is supported. Done means the workflow can exercise both requested dependency states and the test suite completes under Actions.