[SPARK-52255][BUILD] Adds a new environment variable SPARK_RC_COUNT in release script

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

This PR proposes to add `SPARK_RC_COUNT` environment variable to specify the RC count.

### Why are the changes needed?

To make the Spark release fully without developer interaction.

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

No, dev-only.

### How was this patch tested?

Manually tested.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #50975 from HyukjinKwon/SPARK-52255.

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: HyukjinKwon

dev/create-release/do-release-docker.sh

@@ -153,6 +153,7 @@ USER=$USER
 DEBUG_MODE=$DEBUG_MODE
 ANSWER=$ANSWER
 GITHUB_ACTIONS=$GITHUB_ACTIONS
+SPARK_RC_COUNT=$SPARK_RC_COUNT
 EOF
 
 JAVA_VOL=

dev/create-release/release-util.sh

@@ -141,6 +141,9 @@ function get_release_info {
   export RELEASE_VERSION=$(read_config "Release" "$RELEASE_VERSION")
 
   RC_COUNT=$(read_config "RC #" "$RC_COUNT")
+  if [ -n "$SPARK_RC_COUNT" ]; then
+    RC_COUNT=$SPARK_RC_COUNT
+  fi
 
   # Check if the RC already exists, and if re-creating the RC, skip tag creation.
   RELEASE_TAG="v${RELEASE_VERSION}-rc${RC_COUNT}"