Update Rust in builder to 1.69.0

[legoktm]

Status

Ready for review

Description of Changes

First, adjust how Rust is installed in the builder container

Install Rust and rustup in slightly more straightforward directories and
unconditionally add it to $PATH instead of weirdly in /root/.cargo.

This setup is partially based on rust-lang/rustup#1085 (comment),
which is described as unsupported for multiple users but our containers
are single-user.

The plan is to copy this to the development container as well.

Second, and primarily, update Rust in builder to 1.69.0

For the Sequoia work, we're planning to use 1.69.0 so we need to update
the builder to match. This is being done separately since it already
affects a production component (the cryptography wheel) and should be
tested independently.

This also lets us get rid of our index-warming hack since we can use the
new fast sparse protocol instead
(https://blog.rust-lang.org/2023/03/09/Rust-1.68.0.html#cargos-sparse-protocol).

Refs #6817.

Testing

How should the reviewer test this PR?

• CI passes, including deb building and staging build
• Build new debs, deploy on a staging/prod instance:
  • Create new journalist, log in as them.

(AFAIK that's the main place we use cryptography on the server)

Deployment

Any special considerations for deployment? No.

Checklist

• These changes do not require documentation

[cfm]

Thanks for proactively making changes from #6828 (comment). :-)

• CI passes, including deb building and staging build
• Build new debs, deploy on a staging/prod instance:
  • Create new journalist, log in as them.

(AFAIK that's the main place we use cryptography on the server)

I spot a few more places. But a full smoke-test of the Source and Journalist Interfaces checks out just fine!