GH-47063: [Release] Define missing RELEASE_TARBALL (#47064)

### Rationale for this change

`RELEASE_TARBALL` is registered to `GITHUB_ENV` but isn't defined in this context.

### What changes are included in this PR?

Define `RELEASE_TARBARLL`.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* GitHub Issue: #47063

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

.github/workflows/release_candidate.yml

@@ -89,8 +89,9 @@ jobs:
             "dev/release/utils-create-release-tarball.sh ${VERSION} ${RC_NUM}" \
             apache-arrow-${VERSION}.tar.gz
           dev/release/utils-create-release-tarball.sh ${VERSION} ${RC_NUM}
-          echo "RELEASE_TARBALL=apache-arrow-${VERSION}.tar.gz" >> ${GITHUB_ENV}
-          dev/release/utils-generate-checksum.sh "apache-arrow-${VERSION}.tar.gz"
+          RELEASE_TARBALL=apache-arrow-${VERSION}.tar.gz
+          echo "RELEASE_TARBALL=${RELEASE_TARBALL}" >> ${GITHUB_ENV}
+          dev/release/utils-generate-checksum.sh "${RELEASE_TARBALL}"
           if [ -n "${ARROW_GPG_SECRET_KEY}" ]; then
             echo "${ARROW_GPG_SECRET_KEY}" | gpg --import
             gpg \