diff --git a/eng/common/cross/toolchain.cmake b/eng/common/cross/toolchain.cmake
index 561576be9..964610524 100644
--- a/eng/common/cross/toolchain.cmake
+++ b/eng/common/cross/toolchain.cmake
@@ -1,5 +1,12 @@
 set(CROSS_ROOTFS $ENV{ROOTFS_DIR})
 
+# reset platform variables (e.g. cmake 3.25 sets LINUX=1)
+unset(LINUX)
+unset(FREEBSD)
+unset(ILLUMOS)
+unset(ANDROID)
+unset(TIZEN)
+
 set(TARGET_ARCH_NAME $ENV{TARGET_BUILD_ARCH})
 if(EXISTS ${CROSS_ROOTFS}/bin/freebsd-version)
   set(CMAKE_SYSTEM_NAME FreeBSD)
diff --git a/eng/common/templates/steps/source-build.yml b/eng/common/templates/steps/source-build.yml
index 12a8ff94d..4624885e3 100644
--- a/eng/common/templates/steps/source-build.yml
+++ b/eng/common/templates/steps/source-build.yml
@@ -63,6 +63,11 @@ steps:
       targetRidArgs='/p:TargetRid=${{ parameters.platform.targetRID }}'
     fi
 
+    runtimeOsArgs=
+    if [ '${{ parameters.platform.runtimeOS }}' != '' ]; then
+      runtimeOsArgs='/p:RuntimeOS=${{ parameters.platform.runtimeOS }}'
+    fi
+
     publishArgs=
     if [ '${{ parameters.platform.skipPublishValidation }}' != 'true' ]; then
       publishArgs='--publish'
@@ -75,6 +80,7 @@ steps:
       $internalRuntimeDownloadArgs \
       $internalRestoreArgs \
       $targetRidArgs \
+      $runtimeOsArgs \
       /p:SourceBuildNonPortable=${{ parameters.platform.nonPortable }} \
       /p:ArcadeBuildFromSource=true
   displayName: Build