rustc-serialize v0.3.24 contains code that will be rejected by a future version of Rust

[almereyda]

Running the linter (which implies a build), a build or the tests outputs:

warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 6`

As seen in

• https://github.com/CirclesUBI/pathfinder2/actions/runs/4546149224/jobs/8014421497?pr=26#step:7:22
• https://github.com/CirclesUBI/pathfinder2/actions/runs/4546149224/jobs/8014421497?pr=26#step:8:37
• https://github.com/CirclesUBI/pathfinder2/actions/runs/4546149224/jobs/8014421497?pr=26#step:10:36

We will be affected by this, as we are building from the rolling Rust upstream:

pathfinder2/Dockerfile

Line 1 in 4846da2

FROM rust:latest AS build

Implying that

https://github.com/CirclesUBI/pathfinder2/blob/4846da28161055617d790e3ce09613b5c4a528aa/.github/workflows/test.yml#LL29C28-L29C48

also installs Rust in case of the test workflow.

Apparently this manual step could now also be replaced with the upstream supported https://github.com/foundry-rs/foundry-toolchain action. (found via https://github.com/foundry-rs/foundry/#installing-for-ci-in-github-action)

For Dockerfile-based environments, they suggest to build on their upstream image, as per https://github.com/foundry-rs/foundry/#installing-via-docker and https://book.getfoundry.sh/tutorials/foundry-docker