Simplify and speed up the clamav build.

[sengi]

• Use govuk-ruby-builder for clamav. Simplifies the Dockerfile by eliminating boilerplate and speeds up the build by not having to install a bunch of stuff that already ships with govuk-ruby-builder.
• Don't run the clamav Python testsuite; it's super slow and we're building from an official source tarball with a very common config, so it's not worth blowing up asset-manager's build time.
• Add a crude but fast and somewhat effective smoke test for the two clam binaries that asset-manager needs.
• Use the system Rust instead of faffing about installing it ourselves, since we don't anticipate any need to run on PowerPC any time soon.
• Remove a bunch of unneeded Apt packages.
• Remove some unneeded workarounds, e.g. install the clam binaries in the desired directory (which happens to be the default one from the CMakelists.txt anyway) rather than symlinking them afterwards.
• Remove some cough GPLed code cough that was mistakenly copy-pasted into the Dockerfile of our MIT-licensed codebase here.
• Use a valid command for clamdscan so that it works by default rather than requiring brittle configuration on deployment.

Tested: built and ran locally with Docker Desktop, copied configs into /usr/local/etc/*clam*.conf, successfully ran freshclam and scanned some files with clamdscan connecting to clamd via TCP on 127.0.0.1 (which is how we do it in prod).

Rollout: needs manual e2e testing in integration or staging before pushing to prod. (Done — successfully created a document with at attachment and an image in integration via whitehall-admin).