Fixed @woocomerce/classic-assets Watch Command (woocommerce#44627)

The `watch:build` setup in this package does not conform to
the expects format and so the build command was not being
run. This fixes that and ensures it gets executed correctly.

plugins/woocommerce/changelog/fix-classic-asset-watch

@@ -0,0 +1,5 @@
+Significance: patch
+Type: dev
+Comment: This is a tooling-only build change
+
+

plugins/woocommerce/client/legacy/package.json

@@ -13,7 +13,8 @@
 		"build:project": "pnpm --if-present '/^build:project:.*$/'",
 		"build:project:assets": "wireit",
 		"watch:build": "pnpm --if-present --workspace-concurrency=Infinity --filter=\"$npm_package_name...\" --parallel '/^watch:build:project:.*$/'",
-		"watch:build:project": "pnpm build:project --watch"
+		"watch:build:project": "pnpm --if-present run '/^watch:build:project:.*$/'",
+		"watch:build:project:assets": "pnpm build:project --watch"
 	},
 	"devDependencies": {
 		"@types/node": "^16.18.68",