Use ninja if installed for both macOS and Linux

Based on PR feedback, default to using Ninja for native builds on any
Unix platform where ninja is installed, rather than only on macOS.
This aligns with the Mono subtree behavior and provides faster builds
when ninja is available.

Author: steveisok

eng/build.sh

@@ -84,7 +84,7 @@ usage()
   echo "  --gccx.y                   Optional argument to build using gcc version x.y."
   echo "  --portablebuild            Optional argument: set to false to force a non-portable build."
   echo "  --keepnativesymbols        Optional argument: set to true to keep native symbols/debuginfo in generated binaries."
-  echo "  --ninja                    Optional argument: use Ninja instead of Make (default: true on macOS, use --ninja false to disable)."
+  echo "  --ninja                    Optional argument: use Ninja instead of Make (default: true if ninja is installed, use --ninja false to disable)."
   echo "  --pgoinstrument            Optional argument: build PGO-instrumented runtime"
   echo "  --fsanitize                Optional argument: Specify native sanitizers to instrument the native build with. Supported values are: 'address'."
   echo ""
@@ -166,8 +166,8 @@ source $scriptroot/common/native/init-os-and-arch.sh
 
 hostArch=$arch
 
-# Default to using Ninja on macOS for faster builds (can be overridden with --ninja false)
-if [[ "$os" == "osx" ]]; then
+# Default to using Ninja if installed for faster builds (can be overridden with --ninja false)
+if command -v ninja &> /dev/null; then
   useNinja=true
 else
   useNinja=false