GH Actions: use explicit bash shell & other defaults

[MetRonnie]

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#defaultsrunshell

Supported platform	shell parameter	Description	Command run internally
Linux / macOS	unspecified	The default shell on non-Windows platforms. Note that this runs a different command to when bash is specified explicitly. If bash is not found in the path, this is treated as sh.	bash -e {0}
All	bash	The default shell on non-Windows platforms with a fallback to sh. When specifying a bash shell on Windows, the bash shell included with Git for Windows is used.	bash --noprofile --norc -eo pipefail {0}

This was added to the docs in actions/runner#1955; I was unaware that there was a difference between not specifying the default and specifying bash.

We seem to be missing -o pipefail by default. (This change will also introduce --norc, --noprofile; not sure why these are included 🤷)

Tip

If/when we change to using setup-micromamba/miniconda, we will need to provide the full set of options we want, which will have to include -l.

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
• tests, docs, etc not needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Will be revoked by #6655

Looking into how/if micromamba-shell handles this.