### What changes were proposed in this pull request?
Fix regression bug in load-spark-env.cmd with Spark 3.0.0
### Why are the changes needed?
cmd doesn't support set env twice. So set `SPARK_ENV_CMD=%SPARK_CONF_DIR%\%SPARK_ENV_CMD%` doesn't take effect, which caused regression.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Manually tested.
1. Create a spark-env.cmd under conf folder. Inside this, `echo spark-env.cmd`
2. Run old load-spark-env.cmd, nothing printed in the output
2. Run fixed load-spark-env.cmd, `spark-env.cmd` showed in the output.
Closes#29044 from warrenzhu25/32227.
Lead-authored-by: Warren Zhu <zhonzh@microsoft.com>
Co-authored-by: Warren Zhu <warren.zhu25@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>