ci: CI improvements

[geeknoid]

• Save a bunch of lines of mind-numbing GitHub actions YAML.

• Update to latest tool and action versions.

• Stop using the cargo dependency cache, it's redundant with the other caches we have.

• Enable Sccache on two jobs that should've been using it, which should speed those up.

• Switch to using cargo nextest for running our tests which delivers a considerable perf improvement.

• Run cargo-careful as additional prevention against introducing undefined behaviors.

• Stop using cargo-audit since it is completely redundant with cargo-deny which we already run and does more.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (e095f6a) to head (b0375ff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          90       90           
  Lines        6497     6497           
=======================================
  Hits         6497     6497           

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[Copilot]

Pull request overview

This PR significantly refactors the GitHub Actions CI/CD configuration by consolidating repetitive workflow setup into a reusable composite action, while also updating various tool and action versions.

Key changes:

• Introduces a new centralized .github/actions/setup composite action that handles environment configuration, caching, Rust toolchain installation, and cargo tool installation
• Removes the redundant repo-constants.yml workflow and load-constants action in favor of directly parsing constants.toml within the setup action
• Updates multiple tool versions (cargo tools from v0.3.0→v0.4.0, v25.3.1→v26.0.0, etc.) and GitHub Actions versions (checkout v6.0.0→v6.0.1, cache v4.3.0→v5.0.1, etc.)

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
constants.toml	Updated tool versions for cargo utilities and Rust nightly toolchains; improved documentation comments
.github/workflows/repo-constants.yml	Removed - functionality moved to setup action
.github/actions/load-constants/action.yml	Removed - functionality moved to setup action
.github/actions/setup/action.yml	New composite action that consolidates environment setup, constant loading, caching, and tool installation
.github/workflows/nightly.yml	Refactored to use new setup action; updated checkout version
.github/workflows/main.yml	Refactored all jobs to use new setup action; updated multiple action versions; removed redundant env blocks and dependencies

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[psandana]

My only FYI, is that cargo nextest run tests faster, but it runs each test in a subprocess, thus test running on nextest won't conflict each other, while in cargo test they will.

For instance, setting up a global telemetry provider for tracing will affect only a single test on nextest, while it will be set for all tests using cargo test.

Knowing this fact, I still love nextest. The performance, and the UI is simply the best.