Use CDN for ci-caches on download

[Mark-Simulacrum]

This will reduce costs, as well as lays the groundwork for developers to be able
to locally pull the published docker images without needing AWS credentials.

r? @pietroalbini

[Mark-Simulacrum]

I also set the CACHE_DOMAIN variable in azure's public and private libraries; didn't touch GHA at all.

I wanted to also add support for pulling from this cache locally (e.g. when you want to test why android isn't working and don't want to wait for the docker build) but our cache keying appears to use absolute paths today which discouraged me from doing anything there -- seems hard to make that work well. Maybe we can explore relative paths there, but then there's also docker version and such to consider and altogether it seems like the wrong approach.

[pietroalbini]

@Mark-Simulacrum can you set the variable in the public-variables and prod-variables anchors of the GHA configuration?

rust/src/ci/github-actions/ci.yml

Lines 37 to 53 in 3fe4dd2

	- &public-variables
	SCCACHE_BUCKET: rust-lang-gha-caches
	TOOLSTATE_REPO: https://github.com/pietroalbini/rust-toolstate
	- &prod-variables
	SCCACHE_BUCKET: rust-lang-gha-caches
	DEPLOY_BUCKET: rust-lang-gha
	TOOLSTATE_REPO: https://github.com/pietroalbini/rust-toolstate
	TOOLSTATE_ISSUES_API_URL: https://api.github.com/repos/pietroalbini/rust-toolstate/issues
	TOOLSTATE_PUBLISH: 1
	# AWS_SECRET_ACCESS_KEYs are stored in GitHub's secrets storage, named
	# AWS_SECRET_ACCESS_KEY_<keyid>. Including the key id in the name allows to
	# rotate them in a single branch while keeping the old key in another
	# branch, which wouldn't be possible if the key was named with the kind
	# (caches, artifacts...).
	CACHES_AWS_ACCESS_KEY_ID: AKIA46X5W6CZOMUQATD5
	ARTIFACTS_AWS_ACCESS_KEY_ID: AKIA46X5W6CZH5AYXDVF

[Mark-Simulacrum]

Set the variable and ran expansion.

[pietroalbini]

@bors r+

[bors]

📌 Commit 9a4e718 has been approved by pietroalbini

[pietroalbini]

@bors r+ rollup

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: Update transitive dependency to work towards removing syn <1.0 dep #71919

[bors]

📌 Commit 9a4e718 has been approved by pietroalbini