Use mimalloc for musl builds

[jssblck]

I was reminded of this when reading reddit before work this morning: https://nickb.dev/blog/default-musl-allocator-considered-harmful-to-performance/

We do musl builds as part of our current release process, and we care about performance a lot, so...

[Copilot]

Pull Request Overview

This PR integrates mimalloc as the global allocator for musl-based builds to address performance issues with musl's default allocator. The change affects both the hurry and courier packages, which are built for musl targets as part of the release process.

Key changes:

• Added mimalloc as a conditional dependency for musl targets in both hurry and courier packages
• Configured mimalloc as the global allocator when building for musl environments
• Consolidated workspace dependency declarations by moving is_executable and directories to the workspace root

Reviewed Changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 1 comment.

File	Description
packages/hurry/Cargo.toml	Added mimalloc dependency for musl targets; migrated dependencies to workspace
packages/courier/Cargo.toml	Added mimalloc dependency for musl targets
packages/courier/src/main.rs	Set mimalloc as global allocator for musl builds
Cargo.toml	Added workspace-level dependency declarations for is_executable and directories

[elldritch]

What does performance look like before and after this change on your machine?

[jssblck]

What does performance look like before and after this change on your machine?

@elldritch this was more of a proactive change than a reactive one. I've definitely profiled applications in the past where this made a large difference. I can get numbers if you'd like but I do think this is a pretty low-risk high-impact change.

[jssblck]

I plan to revisit this once the CLI is a bit more stable, putting this on ice for now