GH Actions: test conda on min python version

[MetRonnie]

Note

Min Python version for conda is 3.8, not 3.7, due to lack of protobuf build for 3.7. See conda-forge/cylc-flow-feedstock#93

Unfortunately we didn't spot this in #5828 because we were not testing on Python 3.7

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes or tests or docs
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

FYI, I think we can set this as a default to avoid having to configuer it for each step.

https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/setting-a-default-shell-and-working-directory#example-set-the-default-shell-and-working-directory

[oliver-sanders]

There is a second workflow which should also have spotted this, it is tracked on the status page, but it isn't run on a schedule so there error wasn't detected.

https://github.com/cylc/cylc-doc/blob/master/.github/workflows/create-conda-envs.yml