[ci] Manually set rust stable version in CI pipeline

[notmandatory]

Description

This PR sets the rust stable version in the CI pipeline to 1.50.0. This value will need to be updated manually when a new stable version of Rust 🦀 is released. Any clippy or other fixes required by a new stable version should be fixed in the same PR.

Resolves discussion in #282

Notes to the reviewers

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I'm linking the issue being fixed by this PR

[RCasatta]

Sorry for commenting late, maybe it was been better to use:

env:
  RUST_STABLE: 1.50.0

and then ${{ env.RUST_STABLE }} when needed

[notmandatory]

I did try using a global 'env' but it isn't supported across jobs since they run in different containers. So I just hard coded it for now. Maybe it will be possible in the future.