Preserve default Ninja behavior when MSBuild property is unset (#134471)

## Summary

Unset `Ninja` properties were treated as explicitly disabled, overriding
the native build scripts’ default generator selection. Elsewhere in the
clr build, Ninja is treated as the default. Usually this is propagated
from the outer ./build.sh, but when building one of these projects
directly (as trimmer tests do) without explicitly setting
`/p:Ninja=true`, it defaults to Makefiles. This causes the build to fail
if CMakeCache.txt is present after previous build used the default
`Ninja`.

## Changes

- Pass `-ninja false` only when `Ninja=false`.
- Preserve explicit `Ninja=true` behavior.
- Retain the existing Windows-specific MSBuild selection.
- Apply consistently across CoreCLR, corehost, and native libraries.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jtschuster <36744439+jtschuster@users.noreply.github.com>

Author: Copilot

src/coreclr/runtime.proj

@@ -43,7 +43,7 @@
                                    '$(PgoInstrument)' != 'true'"
                                    Include="-enforcepgo" />
       <_CoreClrBuildArg Condition="'$(Ninja)' == 'true' and !$([MSBuild]::IsOsPlatform(Windows))" Include="-ninja" />
-      <_CoreClrBuildArg Condition="'$(Ninja)' != 'true' and !$([MSBuild]::IsOsPlatform(Windows))" Include="-ninja false" />
+      <_CoreClrBuildArg Condition="'$(Ninja)' == 'false' and !$([MSBuild]::IsOsPlatform(Windows))" Include="-ninja false" />
       <_CoreClrBuildArg Condition="'$(Ninja)' == 'false' and $([MSBuild]::IsOsPlatform(Windows))" Include="-msbuild" />
       <_CoreClrBuildArg Condition="'$(PgoInstrument)' == 'true'" Include="-pgoinstrument" />
       <_CoreClrBuildArg Condition="'$(NativeOptimizationDataSupported)' == 'true' and '$(NoPgoOptimize)' != 'true' and '$(PgoInstrument)' != 'true'" Include="-pgodatapath &quot;$(PgoPackagePath)&quot;" />

src/native/corehost/corehost.proj

@@ -85,7 +85,7 @@
       <BuildArgs Condition="'$(CppCompilerAndLinker)' != ''">$(BuildArgs) $(CppCompilerAndLinker)</BuildArgs>
       <BuildArgs Condition="'$(CMakeArgs)' != ''">$(BuildArgs) -cmakeargs "$(CMakeArgs)"</BuildArgs>
       <BuildArgs Condition="'$(Ninja)' == 'true'">$(BuildArgs) -ninja</BuildArgs>
-      <BuildArgs Condition="'$(Ninja)' != 'true'">$(BuildArgs) -ninja false</BuildArgs>
+      <BuildArgs Condition="'$(Ninja)' == 'false'">$(BuildArgs) -ninja false</BuildArgs>
       <BuildArgs>$(BuildArgs) -runtimeflavor $(RuntimeFlavor)</BuildArgs>
       <BuildArgs Condition="'$(EnableNativeSanitizers)' != ''">$(BuildArgs) -fsanitize=$(EnableNativeSanitizers)</BuildArgs>
       <BuildArgs Condition="'$(OfficialBuildId)' != ''">$(BuildArgs) /p:OfficialBuildId="$(OfficialBuildId)"</BuildArgs>

src/native/libs/build-native.proj

@@ -40,7 +40,7 @@
           Condition="!$([MSBuild]::IsOsPlatform(Windows))">
     <PropertyGroup>
       <_BuildNativeArgs Condition="'$(Ninja)' == 'true'">$(_BuildNativeArgs) ninja</_BuildNativeArgs>
-      <_BuildNativeArgs Condition="'$(Ninja)' != 'true'">$(_BuildNativeArgs) ninja false</_BuildNativeArgs>
+      <_BuildNativeArgs Condition="'$(Ninja)' == 'false'">$(_BuildNativeArgs) ninja false</_BuildNativeArgs>
       <!--
         MSBuildNodeCount should a good approximation for how many procs to use for native build, if we find that doesn't work
         then we should consider calling Environment.ProcessorCount