update GitHub Actions CI

[striezel]

The following updates are performed:

• update actions/checkout to v3
• replace unmaintained actions-rs/toolchain by dtolnay/rust-toolchain

Still using the outdated / unmaintained actions will generate several warnings in CI runs, for example in https://github.com/PSeitz/lz4_flex/actions/runs/4843333107:

Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16: actions-rs/toolchain@v1, actions/checkout@v2. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

The PR will get rid of those warnings for actions/checkout and actions-rs/toolchain.

[codecov[bot]]

Codecov Report

Merging #106 (4468375) into main (712d4be) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #106   +/-   ##
=======================================
  Coverage   88.79%   88.79%           
=======================================
  Files          11       11           
  Lines        2195     2195           
=======================================
  Hits         1949     1949           
  Misses        246      246           

[PSeitz]

Thanks!