action: do not re-install ninja

After many years asking, Github runners finally got ninja by
default in April 2025:
- actions/runner-images#11235

So, stop re-installing ninja - at least not unless there is a specific
and documented reason to do so. This saves a little bit of time and
fixes the following warning in hello_world_multiplatform.yaml:

https://github.com/zephyrproject-rtos/zephyr/actions/runs/19853412716
```
build (macos-14)
ninja 1.13.2 is already installed and up-to-date.
To reinstall 1.13.2, run: brew reinstall ninja
```

Do not change the Windows build yet because the build environment is
always much more complicated on Windows, so let's first make sure this
is fine on Linux and macOS.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

action.yml

@@ -96,12 +96,12 @@ runs:
         if [ "${{ runner.os }}" = "Linux" ]; then
           sudo rm -f /var/lib/man-db/auto-update
           sudo apt-get update -y
-          sudo apt-get install -y ninja-build ccache gperf
+          sudo apt-get install -y ccache gperf
           if [ "${{ runner.arch }}" = "X64" ]; then
             sudo apt-get install -y libc6-dev-i386 g++-multilib
           fi
         elif [ "${{ runner.os }}" = "macOS" ]; then
-          brew install ninja ccache qemu dtc gperf
+          brew install ccache qemu dtc gperf
         elif [ "${{ runner.os }}" = "Windows" ]; then
           choco feature enable -n allowGlobalConfirmation
           choco install ninja wget gperf