Update github workflow

[pbor]

• switch to dtolnay's toolchain action
• use cargo-llvm-cov instead of tarpaulin

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[fabiosky]

Closed in favor of https://github.com/aws/dcv-color-primitives/pull/85