[CI] Add CI to validate VCX MSRV toolchain

[JamesKEbert]

As discussed in #1324, we should add CI to validate that VCX works as expected in both our target Rust version as well as our Minimum Supported Rust Version (MSRV). If the check fails in the future, it will be a signal to either raise our MSRV or adjust the incoming PR.

This may be as simple as "cargo check --all-features (or just check-workspace perhaps?)".