[SPARK-32434][CORE][FOLLOW-UP] Fix load-spark-env.cmd to be able to run in Windows properly

### What changes were proposed in this pull request?

This PR is basically a followup of SPARK-26132 and SPARK-32434. You can't define an environment variable within an-if to use it within the block. See also https://superuser.com/questions/78496/variables-in-batch-file-not-being-set-when-inside-if

### Why are the changes needed?

For Windows users to use Spark and fix the build in AppVeyor.

### Does this PR introduce _any_ user-facing change?

No, it's only in unreleased branches.

### How was this patch tested?

Manually tested on a local Windows machine, and AppVeyor build at HyukjinKwon#13. See https://ci.appveyor.com/project/HyukjinKwon/spark/builds/34316409

Closes #29254 from HyukjinKwon/SPARK-32434.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HyukjinKwon

bin/load-spark-env.cmd

@@ -22,7 +22,7 @@ rem spark-env.cmd is loaded from SPARK_CONF_DIR if set, or within the current di
 rem conf\ subdirectory.
 
 set SPARK_ENV_CMD=spark-env.cmd
-if [%SPARK_ENV_LOADED%] == [] (
+if not defined SPARK_ENV_LOADED (
   set SPARK_ENV_LOADED=1
 
   if [%SPARK_CONF_DIR%] == [] (
@@ -37,18 +37,19 @@ if [%SPARK_ENV_LOADED%] == [] (
 
 rem Setting SPARK_SCALA_VERSION if not already set.
 
-if [%SPARK_SCALA_VERSION%] == [] (
-  set SCALA_VERSION_1=2.13
-  set SCALA_VERSION_2=2.12
+set SCALA_VERSION_1=2.13
+set SCALA_VERSION_2=2.12
 
-  set ASSEMBLY_DIR1=%SPARK_HOME%\assembly\target\scala-%SCALA_VERSION_1%
-  set ASSEMBLY_DIR2=%SPARK_HOME%\assembly\target\scala-%SCALA_VERSION_2%
-  set ENV_VARIABLE_DOC=https://spark.apache.org/docs/latest/configuration.html#environment-variables
+set ASSEMBLY_DIR1=%SPARK_HOME%\assembly\target\scala-%SCALA_VERSION_1%
+set ASSEMBLY_DIR2=%SPARK_HOME%\assembly\target\scala-%SCALA_VERSION_2%
+set ENV_VARIABLE_DOC=https://spark.apache.org/docs/latest/configuration.html#environment-variables
+
+if not defined SPARK_SCALA_VERSION (
   if exist %ASSEMBLY_DIR2% if exist %ASSEMBLY_DIR1% (
-    echo "Presence of build for multiple Scala versions detected (%ASSEMBLY_DIR1% and %ASSEMBLY_DIR2%)."
-    echo "Remove one of them or, set SPARK_SCALA_VERSION=%SCALA_VERSION_1% in %SPARK_ENV_CMD%."
-    echo "Visit %ENV_VARIABLE_DOC% for more details about setting environment variables in spark-env.cmd."
-    echo "Either clean one of them or, set SPARK_SCALA_VERSION in spark-env.cmd."
+    echo Presence of build for multiple Scala versions detected ^(%ASSEMBLY_DIR1% and %ASSEMBLY_DIR2%^).
+    echo Remove one of them or, set SPARK_SCALA_VERSION=%SCALA_VERSION_1% in spark-env.cmd.
+    echo Visit %ENV_VARIABLE_DOC% for more details about setting environment variables in spark-env.cmd.
+    echo Either clean one of them or, set SPARK_SCALA_VERSION in spark-env.cmd.
     exit 1
   )
   if exist %ASSEMBLY_DIR1% (