check if GitHub CLI is installed and react accordingly

script/release

@@ -11,11 +11,18 @@ if npm whoami &>/dev/null; then
   npm run release
 
   # The default branch is protected.
-  # Raise a chore PR to bump `version` in `package.json`.
+  # Push commit to new branch.
   NEW_VERSION=$(git log -1 --pretty=%B)
   git checkout -b "release-${NEW_VERSION}"
   git push -u origin "release-${NEW_VERSION}"
-  gh pr create --title "chore: ${NEW_VERSION}" --body ""
+
+  if ! command -v gh &> /dev/null
+  then
+      echo "GitHub CLI not found. Please raise PR manually."
+  else
+    # Raise a chore PR to bump `version` in `package.json`.
+    gh pr create --title "chore: ${NEW_VERSION}" --body ""
+  fi
 else
   echo "You are not logged into npm!"
   echo "  Run 'npm login' first!"