ci: simplified CI workflow and removed unnecessary miri script

[alejandro-vaz]

this PR tweaks the CI workflow to only install components on the fly when needed instead of for all versions

additionally, it installs miri directly

it doesn't seem to be substantially faster, but it looks like there are some improvements on the nightly job

the old miri script was deleted

[jdm]

Just to be clear about some coverage that is being removed:

• we no longer explicitly verify that --no-default-features works, but we don't actually have default features
• we will fail CI if miri is unavailable for any given nightly toolchain that we use
• we now run benchmarks with all features enabled instead of the default features
• we only run miri with all features enabled

I feel like these are reasonable simplifications, but I just wanted to call them out.

[alejandro-vaz]

yeah, those are reasonable simplifications

• not having default features means we can simply remove that step
• miri is usually always shipped with nightly these days
• running benchmarks with all features allows us to benchmark those features if necessary (will work on it with add integration tests #440)
• same with miri for all features enabled, it allows us to test those features