Use Posit bioc mirror to download config.yaml

[jeroen]

Some actions fail randomly because the upstream Bioc servers are flaky: https://github.com/jeroen/toml/actions/runs/18412341351/job/52467715182

Maybe we should just get this file from the posit mirror: https://bioconductor.posit.co/config.yaml

[gaborcsardi]

Is this still happening?

Nevertheless, I think this is something to fix in pkgcache. We already have r-lib/pkgcache#134 so we'll handle it there.

[github-actions]

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