Pass --watcher only when the build watches

CHANGELOG.md

@@ -1,6 +1,9 @@
 main
 ------
 
+* Pass `--watcher` to `ember build` only when the build watches. The flag
+  names the backend that watches the file system, so it did nothing for a
+  one-off build
 * Build a Vite-based application with `vite build`, the command its own
   `build` script runs, rather than with `ember build`. `ember build --help`
   calls itself a "Vestigial command in Vite-based projects" and points at

lib/ember_cli/command.rb

@@ -86,11 +86,14 @@ def vite_build
     # Builds the application with `ember build`, which watches for changes
     # when asked. A Vite-based project has no equivalent: `ember build` there
     # refuses `--watch`, and its development server watches instead.
+    #
+    # `--watcher` names the backend that watches, so it rides along only
+    # when the build watches.
     def ember_build(watch: false)
       line = Terrapin::CommandLine.new(paths.ember, [
         "build",
         ("--watch" if watch),
-        ("--watcher :watcher" if process_watcher),
+        ("--watcher :watcher" if watch && process_watcher),
         ("--silent" if silent?),
         "--environment :environment",
         "--output-path :output_path",

spec/lib/ember_cli/command_spec.rb

@@ -77,6 +77,15 @@
       end
     end
 
+    context "when not configured to watch" do
+      it "omits the `--watcher` flag" do
+        paths = build_paths
+        command = build_command(paths: paths, options: { watcher: "foo" })
+
+        expect(command.build).not_to match(/--watcher/)
+      end
+    end
+
     context "when configured to watch" do
       it "includes the `--watch` flag" do
         paths = build_paths