ci: use GCS instead of S3 to download binaries

[rail]

Previously, CI used S3 to download the binaries. Now that we are moving the primary location to GCS and will stop uploading to s3 at some point, it's time to start using GCS for this operation.

Part of RE-342
Release note: None

[cockroach-teamcity]

This change is 

[rail]

bors r=rickystewart

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from c9adfd9 to blathers/backport-release-22.1-92073: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.x failed. See errors above.

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