make verify-channel.sh compatible with macOS

src/ci/scripts/verify-channel.sh

@@ -8,21 +8,31 @@ IFS=$'\n\t'
 
 source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 
-declare -A CHANNEL_BRANCH
-CHANNEL_BRANCH["nightly"]="master"
-CHANNEL_BRANCH["beta"]="beta"
-CHANNEL_BRANCH["stable"]="stable"
-
 if isCiBranch auto || isCiBranch try; then
     echo "channel verification is only executed on PR builds"
     exit
 fi
 
 channel=$(cat "$(ciCheckoutPath)/src/ci/channel")
+case "${channel}" in
+    nightly)
+        channel_branch="master"
+        ;;
+    beta)
+        channel_branch="beta"
+        ;;
+    stable)
+        channel_branch="stable"
+        ;;
+    *)
+        echo "error: unknown channel defined in src/ci/channel: ${channel}"
+        exit 1
+esac
+
 branch="$(ciBaseBranch)"
-if [[ "${branch}" != "${CHANNEL_BRANCH[$channel]}" ]]; then
+if [[ "${branch}" != "${channel_branch}" ]]; then
     echo "error: PRs changing the \`${channel}\` channel should be sent to the \
-\`${CHANNEL_BRANCH[$channel]}\` branch!"
+\`${channel_branch}\` branch!"
 
     exit 1
 fi