Set UBUNTU_DEFAULT_PLATFORM_ID to "ubuntu.20.04"

CMakeLists.txt

@@ -646,14 +646,15 @@ IF ( IS_UBUNTU )
   SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "${LSB_RELEASE_ID_SHORT}.${LSB_RELEASE_RELEASE_SHORT}-${NUGET_NATIVE_PLATFORM_ID_ARCH}")
   MESSAGE(STATUS "NUGET_NATIVE_PLATFORM_ID_UBUNTU: ${NUGET_NATIVE_PLATFORM_ID_UBUNTU}")
 ELSE()
+  SET(UBUNTU_DEFAULT_PLATFORM_ID "ubuntu.20.04")
   IF (HAVE_LINUX_X86)
-    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "ubuntu-x86")
+    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "${UBUNTU_DEFAULT_PLATFORM_ID}-x86")
   ENDIF()
   IF (HAVE_LINUX_X64)
-    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "ubuntu-x64")
+    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "${UBUNTU_DEFAULT_PLATFORM_ID}-x64")
   ENDIF()
   IF (HAVE_LINUX_ARM64)
-    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "ubuntu-arm64")
+    SET(NUGET_NATIVE_PLATFORM_ID_UBUNTU "${UBUNTU_DEFAULT_PLATFORM_ID}-arm64")
   ENDIF()
 ENDIF()
 STRING(TOLOWER "${NUGET_NATIVE_PLATFORM_ID_UBUNTU}" NUGET_NATIVE_PLATFORM_ID_UBUNTU)