Update CI actions: replace `actions-rs/toolchain` with `dtolnay/rust-toolchain`, update `checkout` action to v3

[mmagician]

Description

CI jobs are failing for apparently no reason in this repo. Maybe related: https://github.com/actions/runner/issues/2468?

I tried to update some workflows, unfortunately no success on my fork, but won't hurt to have actions which are maintained. In follow-up PRs I'll update the rest of the unmaintained actions and also apply the same changes to algebra & curves repos.

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Before we can merge this PR, please make sure that all the following items have been checked off. If any of the checklist items are not applicable, please leave them but write a little note why.

• [ ] Targeted PR against correct branch (master)
• [ ] Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• [ ] Wrote unit tests
• [ ] Updated relevant documentation in the code
• [ ] Added a relevant changelog entry to the Pending section in CHANGELOG.md
• [ ] Re-reviewed Files changed in the Github PR explorer

[Pratyush]

I think the test is failing because the curve operations are too expensive now. This could be because our tests got more expensive, or because the github machines got weaker, or some combination of the two. We can remove the failing CI test if necessary.

[mmagician]

Yeah that seems to have done it, CI in my fork continues after removingmnt6_753 tests: https://github.com/mmagician/r1cs-std/pull/4. Thanks @Pratyush