fix(build): invalidate artifact and workspace caches on build config changes

[aoright]

Description

Modifications to [package.build.config] (such as backend-specific compiler flags, ignore flags, or custom mappings) previously invalidated the backend metadata cache, but ArtifactCacheKey and WorkspaceKey omitted the build configuration hash. Consequently, running pixi install after editing [package.build.config] reused stale built packages from the artifact cache without triggering a rebuild.

This PR:

1. Derives Hash on ConfigurationHash so it can be hashed into cache keys.
2. Returns configuration_hash from resolve_backend_identity alongside the backend identifier string.
3. Incorporates configuration_hash into compute_artifact_cache_key and compute_workspace_key, ensuring changes to [package.build.config] invalidate cached artifacts and workspace scratch directories.

How Has This Been Tested?

• Added integration test test_package_rebuilt_across_sessions_when_build_config_modified in crates/pixi_command_dispatcher/tests/integration/main.rs:
  • Simulates a multi-package workspace across two CLI sessions.
  • Modifies [package.build.config] in one package without altering source files.
  • Verifies that only the package with modified build configuration is queued for rebuild, leaving unchanged packages intact.
• Ran cargo test -p pixi_command_dispatcher test_package_rebuilt_across_sessions_when_build_config_modified.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.