ci: speedup & reduce cache storage

[vilgotf]

Twilight's CI cache regurarely exceeds the maximum provided 10GB storage by GitHub, discarding the oldest caches. The resulting cache misses increases our CI time and wastes GitHub's resources.

By inlining the min-vers CI job into MSRV we reduce it's risk of becoming stale (running dependant jobs quicker) as the dependencies are frozen and also remove an almost duplicate 300MB cache.
By excluding the book from the doctest CI no longer needs to recompile Twilight due to the book's dependencies and this should also reduce this job's cache size.

[vilgotf]

It's impossible to run min-vers with all-features due to itoa 0.4 (pulled in by http 0.2, pulled in by hyper & tungstenite) having a i128 feature which enables a nightly feature (we're running an old stable version, so nightly features are not allowed). Can maybe unify msrv & min-vers after rust-lang/cargo#11688 lands.

I'll revert these changes.

[vilgotf]

Test job:

• prev: 438770249B (~418MB)
• now: 383544825B (~366MB)

[spring4175]

Thanks!