CI: fix tests

[JEnoch]

Since recent bump of some dependencies, the test were failing with such error:

error: package `zerofrom v0.1.6` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.75.0
Either upgrade to rustc 1.81 or newer, or use
cargo update zerofrom@0.1.6 --precise ver
where `ver` is the latest version of `zerofrom` supporting rustc 1.75.0

This PR makes sure that in CI the tests are built with the same Rust compiler and pinned dependencies than the plugin.
For this, it just copies the Cargo.lock and rust-toolchain.toml from root dir.

[github-actions]

PR missing one of the required labels: {'internal', 'documentation', 'enhancement', 'dependencies', 'bug', 'new feature', 'breaking-change'}