[25.0] Set GALAXY_CONFIG_FILE env var if starting handler with -c

[mvdbeek]

I think this should fix #20307
I had previously already tried to fix this via #18819, but this seems to be back in 25.0. It's a bit of a brute-force method, but it looks like once we build the celery_app that's the one the tasks get bound to. We probably could change the decorator to delay celery_app construction until the first use, this should also always work.

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[jdavcs]

Let's try this! 🤞

[github-actions]

This PR was merged without a "kind/" label, please correct.