Update toolchain to stable version

[doraeric]

Can we update the rust-toolchain to a stable version?
It's a nightly version and haven't been updated in 2 years.

It would be more convenient to use a stable version for CI/CD purpose.
Additionally, we can avoid wasting disk space on toolchains that are seldom used.

[mert-kurttutan]

Yeap, I cannot see any nightly feature being used in the project.
I would provide PR, if needed