ci: test against main and devel weekly

[lgarrison]

Describe your changes

We want to make sure that Gala remains compatible with EXP/devel, so we'll schedule our devel tests (which use EXP/devel) to run weekly. Tests in our main continue to run weekly against a tagged EXP version.

To my/Claude's understanding, scheduled GitHub tests only run on the default branch. So what this PR does is set up the default branch weekly tests as a matrix of [main, devel] and check out the appropriate one once the run has kicked off.

Follow-up to #593.

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Relatedly: @adrn thanks for restoring the devel branch. I think you can add a branch protection rule to prevent it from being deleted again: https://github.com/adrn/gala/settings/rules/new?target=branch

(Apologies if you already did so; gh ruleset check says it's not protected, but I'm not sure if I have permission to see all rulesets.)

Checklist

• Did you add tests?
• Did you add documentation for your changes?
• Did you reference any relevant issues?
• Did you add a changelog entry? (see CHANGES.rst)
• Are the CI tests passing?
• Is the milestone set?

[lgarrison]

This should be pretty quick to review. I think CI is green; the ReadTheDocs run may just need to be restarted.

[adrn]

Aha, nice - this looks good. Thanks @lgarrison ! RTD build stall is unrelated.