suggest building with ninja on MacOS in README.md (required for Swift)

[stevengj]

On MacOS, the build instructions don't work because they default to using make, which does not support Swift components. I get an error like:

-- Configuring macOS integration
CMake Error at /opt/homebrew/share/cmake/Modules/CMakeDetermineSwiftCompiler.cmake:52 (message):
  Swift language not supported by "Unix Makefiles" generator
Call Stack (most recent call first):
  src/macos_integration/CMakeLists.txt:31 (enable_language)


CMake Error: CMAKE_Swift_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!
CMake Error at build.cmake:237 (message):
  Configure step failed with status 1.  See output above for details.

It seems that you need to use ninja to build, instead, which is enabled by appending -G Ninja to the cmake command, but this is not obvious from the error message.

This PR updates the README.md file to suggest appending -G Ninja, at least on MacOS.

• I signed the CLA — user stevengj
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request adds a macOS-specific documentation note to the README.md file's Release Build section. The change instructs users to append the -G Ninja flag to their build command when compiling on macOS, and directs them to install the Ninja build tool if it is not already present on their system. The update provides guidance for developers building Swift components as part of the MuseScore release process.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding macOS-specific build instructions recommending the Ninja generator for Swift support in README.md.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description is comprehensive and mostly follows the template structure, including signed CLA, descriptive title, commit message references, and no unnecessary changes verified.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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