Fixes #5898 Adds latest cryptography

[kushaldas]

Status

Ready for review.

Description of Changes

Fixes #5898

This commit brings in the rust compiler toolchain during the package
build and also updates the cryptography package to the latest 3.4.7

For my old T470 laptop, the build time is 13m12.645s. Need someone else to record the time
on their system.

Testing

• Adds Rust to the builder image #5966 needs to be merged first and then a new builder image needs to be pushed.
• Need to get the diff-review before this PR can be merged. https://github.com/freedomofpress/securedrop-debian-packaging/wiki/cryptography-3.2.1-to-3.4.7
• CI should be green.

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you added or removed a file deployed with the application:

• I have updated AppArmor rules to include the change

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[conorsch]

Regarding the run time, locally I'm seeing

real	14m3.100s
user	13m31.686s
sys	1m36.340s

for make build-debs. Hardly a prohibitively long build time—debs usually take 10-15 minutes on my machine.

[zenmonkeykstop]

running docker system prune && make clean && time make build-debs, I get 10m12s for develop and 12m38s for this branch.

[rmol]

👍 The securedrop-app-code contains a functioning cryptography 3.4.7; I tested staging manually and with make testinfra and all looks normal.