CI | Split workflows on dedicated files

[EstebanBorai]

To improve development experience and workflow logs on GitHub UI, the ci workflow is splitted into different files:

• build.yml: Runs cargo build on stable, beta and nightly
• clippy.yml: Runs cargo clippy on stable, beta and nightly
• fmt.yml: Runs cargo fmt on stable, beta and nightly
• test.yml: Runs cargo test on stable, beta and nightly

[rafacouto]

It seems fmt is not found on beta and nightly channels... "plus syntax" issue? https://stackoverflow.com/questions/22758779/is-it-possible-to-have-multiple-coexisting-rust-installations :thinking: