Fix node compilation

[ebma]

The node compilation fails with

$ cargo build --release --quiet
⚡ Found 3 strongly connected components which includes at least one cycle each
cycle(001) ∈ α: DisputeCoordinator ~~{"DisputeDistributionMessage"}~~> DisputeDistribution ~~{"DisputeCoordinatorMessage"}~~>  *
cycle(002) ∈ β: CandidateBacking ~~{"CollatorProtocolMessage"}~~> CollatorProtocol ~~{"CandidateBackingMessage"}~~>  *
cycle(003) ∈ γ: NetworkBridgeRx ~~{"GossipSupportMessage"}~~> GossipSupport ~~{"NetworkBridgeRxMessage"}~~>  *
error: failed to run custom build command for `amplitude-runtime v1.6.0 (/app/prodbuild/pendulum/runtime/amplitude)`
Caused by:
  process didn't exit successfully: `/app/prodbuild/pendulum/target/release/build/amplitude-runtime-[49](https://gitlab.com/pendulumchain/compilation/-/jobs/9011102737#L49)26c6a6f1ea772d/build-script-build` (exit status: 1)
  --- stderr
  Cannot compile the WASM runtime: no standard library sources found!
  You can install them with `rustup component add rust-src` if you're using `rustup`.
error: failed to run custom build command for `pendulum-runtime v1.6.0 (/app/prodbuild/pendulum/runtime/pendulum)`
Caused by:
  process didn't exit successfully: `/app/prodbuild/pendulum/target/release/build/pendulum-runtime-75d6ad216142faa9/build-script-build` (exit status: 1)
  --- stderr
  Cannot compile the WASM runtime: no standard library sources found!
  You can install them with `rustup component add rust-src` if you're using `rustup`.
error: failed to run custom build command for `foucoco-runtime v1.6.0 (/app/prodbuild/pendulum/runtime/foucoco)`
Caused by:
  process didn't exit successfully: `/app/prodbuild/pendulum/target/release/build/foucoco-runtime-06f4fcc6ef2[56](https://gitlab.com/pendulumchain/compilation/-/jobs/9011102737#L56)f6f/build-script-build` (exit status: 1)
  --- stderr
  Cannot compile the WASM runtime: no standard library sources found!
  You can install them with `rustup component add rust-src` if you're using `rustup`.

To fix it, we add rust-src to the components field of the rust-toolchain.toml.