audacity: select native Qt tools when cross compiling

[johnrichardrinehart]

Depends on #561181 for Qt Declarative to provide Qt Quick when cross-compiled.

Description of changes

Use the executable output of native Qt Tools instead of its development output. The development output propagates native Qt libraries and triggers the mismatched-Qt setup-hook error when target Qt libraries are also present.

For cross builds, supply QT_HOST_PATH and the native QML, Quick, and ShaderTools CMake tool-package directories. Keep target Qt libraries in buildInputs. Native Linux and Darwin builds do not receive these cross-only CMake flags.

This follows cross-build verification of #559623. No changes to Audacity 3.

Verification

• On an aarch64-linux builder, cross-compiled Audacity 4.0.0 for x86_64-linux using an overlay with these changes and the Qt fixes from qt6.qtdeclarative: fix host tools for cross compilation #561181.
• Confirmed the output ELF machine type is x86_64.
• Copied its closure to the local x86_64-linux workstation. --long-version and --plugin-registration-self-test both exited 0 with QT_QPA_PLATFORM=offscreen; Nyquist resources were found.
• Evaluated native x86_64-linux, native aarch64-darwin, and aarch64-linux to x86_64-linux configurations.
• Formatting passed.

The successful overlay also set Qt6QuickCompilerTools_DIR; CMake reported it unused, so that setting is omitted here. The native configurations were evaluated, not rebuilt.

AI assistance was used to investigate, implement, and validate this change.