Verify `.cargo/config.toml` is used in CI

[hildebrandmw]

PR #692 added preliminary CI, but it appears that .cargo/config.toml might not be pulled in correctly by CI. The test tolerance for the dynamic PQ distance computer needed to be relaxed, which can only be reproduced locally when setting the build CPU as x86-64 instead of the x86-64-v3 override present in .cargo/config.toml.

It's probably a good idea anyways for our tests to pass when compiling for ancient x86 CPUs, but building properly for x86-64-v3 may help our CI times a little.