[SPARK-51344] Fix ENV key value format in *.template

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

This PR aims to fix `ENV` key value format in `*.template` in order to generate the future Dockerfiles correctly.

### Why are the changes needed?

To follow the Docker guideline to fix the following legacy format.
- https://docs.docker.com/reference/build-checks/legacy-key-value-format/

```
    - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format
```

We already fixed Apache Spark Dockerfiles in the main repository.
- apache/spark#47357

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

No. This removes a warning for now.

### How was this patch tested?

Manual review.

Closes #82 from dongjoon-hyun/SPARK-51344.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Dockerfile.template

@@ -72,7 +72,7 @@ RUN set -ex; \
 
 COPY entrypoint.sh /opt/
 
-ENV SPARK_HOME /opt/spark
+ENV SPARK_HOME=/opt/spark
 
 WORKDIR /opt/spark/work-dir
 

r-python.template

@@ -29,7 +29,7 @@ RUN set -ex; \
     rm -rf /var/lib/apt/lists/*
 {%- if HAVE_R %}
 
-ENV R_HOME /usr/lib/R
+ENV R_HOME=/usr/lib/R
 {%- endif %}
 
 USER spark