circleci: prepare build matrix

[q9f]

• renamed test-executor to linux-executor
• updated linux-executor image to ubuntu-2004:202107-02
• renaned mac-executor to macos-executor
• updated xcode version to 13.0.0
• renamed install_rust command to install_rust_linux
• added install_rust_macos command
• renamed rust_setup to set_rustc_stable
• added set_rustc_nightly
• added linux-build-stable job
• renamed nightly-build-check job to linux-build-nightly
• renamed msrv-build-check job to linux-build-msrv
• renamed macos-build-check job to macos-build-stable
• renamed setup_test to test-checks
• renamed build-check to build-checks
• renamed daily-checks to nightly-checks
• removed todos and unused stuff and stuff we shouldn't ignore

[ec2]

Not sure if we need the MacOS build tests 🤔

[q9f]

Not sure if we need the MacOS build tests thinking

Yesterday you said, you don't know if it's building on macos.

I can also remove macos nightly and just keep stable. But one job on macos should be in place, don't you think?

[ec2]

Not sure if we need the MacOS build tests thinking

Yesterday you said, you don't know if it's building on macos.

I can also remove macos nightly and just keep stable. But one job on macos should be in place, don't you think?

Yeah, I would remove MacOS Nightly and we can keep the stable build.

[ec2]

LGTM. Just remove MacOS nightly as discussed.

[q9f]

I removed mac nightly in 33c0603 and decided to restore msrv of 1.52.1 for now.