ci: limit disk usage

[evilrobot-01]

A few attempts to limit the disk usage during CI tests:

• Sets the default resolver for the virtual workspace as per https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html, mostly to silence the build warnings:

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest

• disables debug info for the test profile (debatable on whether this should be done)
• removes nightly from the setup-worker template
• uses Swatinem/rust-cache for 'smart rust caching', along with disabling incremental builds.

Based on suggestions from https://matklad.github.io/2021/09/04/fast-rust-builds.html#CI-Caching and https://matklad.github.io/2021/09/04/fast-rust-builds.html#CI-Workflow.

PS - caches are now 1.1gb (release) and 1.5gb (debug), whereas previously the debug cache was ~4.5gb.