Use RELEASE_VERSION instead of BUILDKITE_RELEASE_VERSION

The `BUILDKITE_` prefix is a workaround required only on iOS.
We could adopt it for consistency here, too, but it's best to avoid it
for two reasons:

1. Being a workaround, one would hope to eventually remove it
2. The less coupled we are with the particular CI implementation details
   (i.e. "Buildkite" in the name) the better

.buildkite/commands/checkout-release-branch.sh

@@ -1,14 +1,12 @@
 #!/bin/bash -eu
 
-# We expect BUILDKITE_RELEASE_VERSION to be as an environment variable, e.g. by the automation that triggers the build on Buildkite.
-# It must use the `BUILDKITE_` prefix to be passed to the agent due to how `hostmgr` works, in case this runs on a Mac agents.
-if [[ -z "${BUILDKITE_RELEASE_VERSION}" ]]; then
-    echo "BUILDKITE_RELEASE_VERSION is not set."
+if [[ -z "${RELEASE_VERSION}" ]]; then
+    echo "RELEASE_VERSION is not set."
     exit 1
 fi
 
 # Buildkite, by default, checks out a specific commit.
 # For many release actions, we need to be on a release branch instead.
-BRANCH_NAME="release/${BUILDKITE_RELEASE_VERSION}"
+BRANCH_NAME="release/${RELEASE_VERSION}"
 git fetch origin "$BRANCH_NAME"
 git checkout "$BRANCH_NAME"