Check env-var UNITY_ROOT_DIR as well during build (#455)

Currently the build script would automatically find the latest version of Unity unless it is passed with `-unity_root=` parameter.

This change make sure that the build script also check environment variable UNITY_ROOT_DIR before searching for the latest version of Unity.

Author: chkuang-g

cmake/FindUnity.cmake

@@ -20,18 +20,23 @@ if (NOT EXISTS "${UNITY_ROOT_DIR}")
   # Make our best attempt to find the latest unity installed on the system.
   # Note that Unity <=2018 include mono 2.x which causes compilation errors.
   # Minimum supported version of mono is 3.5.
-  if(CMAKE_HOST_WIN32)
-    set(editordir "C:/Program Files/Unity/Hub/Editor")
-  elseif(CMAKE_HOST_APPLE)
-    set(editordir "/Applications/Unity/Hub/Editor")
-  elseif(CMAKE_HOST_UNIX) # Linux
-    set(editordir "$ENV{HOME}/Unity/Hub/Editor")
+  if (EXISTS "$ENV{UNITY_ROOT_DIR}")
+    set(UNITY_ROOT_DIR "$ENV{UNITY_ROOT_DIR}")
+    message(STATUS "Detected Unity version from env-var 'UNITY_ROOT_DIR': ${UNITY_ROOT_DIR}")
+  else()
+    if(CMAKE_HOST_WIN32)
+        set(editordir "C:/Program Files/Unity/Hub/Editor")
+    elseif(CMAKE_HOST_APPLE)
+        set(editordir "/Applications/Unity/Hub/Editor")
+    elseif(CMAKE_HOST_UNIX) # Linux
+        set(editordir "$ENV{HOME}/Unity/Hub/Editor")
+    endif()
+    file(GLOB unity_versions RELATIVE ${editordir} ${editordir}/*)
+    list(SORT unity_versions ORDER DESCENDING)
+    list(GET unity_versions 0 latest_unity_version)
+    set(UNITY_ROOT_DIR ${editordir}/${latest_unity_version})
+    message(STATUS "Auto detected latest Unity version: ${UNITY_ROOT_DIR}")
   endif()
-  file(GLOB unity_versions RELATIVE ${editordir} ${editordir}/*)
-  list(SORT unity_versions ORDER DESCENDING)
-  list(GET unity_versions 0 latest_unity_version)
-  set(UNITY_ROOT_DIR ${editordir}/${latest_unity_version})
-  message(STATUS "Auto detected latest Unity version: ${UNITY_ROOT_DIR}")
 endif()
 
 if(FIREBASE_INCLUDE_UNITY)
@@ -95,7 +100,7 @@ if(FIREBASE_INCLUDE_UNITY)
 
   message(STATUS "UNITY_MONO_EXE is ${UNITY_MONO_EXE}")
   message(STATUS "UNITY_CSHARP_BUILD_EXE is ${UNITY_CSHARP_BUILD_EXE}")
-    
+
   if(NOT EXISTS UNITY_MONO_EXE OR
     NOT EXISTS UNITY_CSHARP_BUILD_EXE)
     set(FIND_TOOL_OPTIONS