Make "cargo-semver-checks" action faster

[Philippe-Cholet]

"cargo-semver-checks-action" previously took 2mn30s in CI which sure is not that much but is still slower than all the rest. It was slowly (PR after PR) driving me nuts.
After installing cargo-semver-checks locally, I witnessed that run the binary was fast. I eventually looked at the CI log and I thought the installation of the toolchain could be the bottleneck.
For it (and other things), the semver-checks action uses "actions-rs/core" which has been recently archived and is therefore unmaintained and might be not as performant as alternatives.

So here, I install it with dtolnay/rust-toolchain@stable (same as other jobs) and set the toolchain to "manual" for "cargo-semver-checks-action". And now it's 🚀blazingly fast🔥!

tldr; CI is overall twice faster.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (6814180) to head (e3c849f).
Report is 90 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
+ Coverage   94.38%   94.55%   +0.17%     
==========================================
  Files          48       49       +1     
  Lines        6665     7002     +337     
==========================================
+ Hits         6291     6621     +330     
- Misses        374      381       +7     

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