github: Stop manually installing ninja

Since https://github.com/actions/runner-images/pull/11706,
https://github.com/actions/runner-images/pull/11707 and
https://github.com/actions/runner-images/pull/11737, ninja is
preinstalled in the Ubuntu, macOS and x86 Windows runner images.
(See https://github.com/actions/runner-images/issues/11391 for the
overall issue where this was finally agreed to be installed.)

It is still missing in the windows-11-arm images though, so keep
installing it in any jobs running on Windows.

.github/workflows/msvcrt.yml

@@ -75,7 +75,6 @@ jobs:
           SOURCE_DATE_EPOCH: ${{needs.prepare.outputs.COMMIT_DATE_UNIX}}
           BUILD_DATE: ${{needs.prepare.outputs.BUILD_DATE}}
         run: |
-          sudo apt-get update && sudo apt-get install ninja-build
           ./build-all.sh $(pwd)/install/llvm-mingw --no-tools --wipe-runtimes --with-default-msvcrt=msvcrt
           .github/workflows/store-version.sh install/llvm-mingw/versions.txt
           cd install
@@ -119,7 +118,7 @@ jobs:
           PYTHON_VERSION_MINGW: ${{needs.prepare.outputs.PYTHON_VERSION_MINGW}}
           TAG: ${{needs.prepare.outputs.TAG}}
         run: |
-          sudo apt-get update && sudo apt-get install libltdl-dev swig autoconf-archive ninja-build
+          sudo apt-get update && sudo apt-get install libltdl-dev swig autoconf-archive
           ./build-cross-tools.sh /opt/llvm-mingw $(pwd)/install/llvm-mingw ${{matrix.arch}} --with-python
           .github/workflows/store-version.sh install/llvm-mingw/versions.txt
           cd install