build: unify YARN_VERSION variable usage and ensure CI uses yarn not npm (#18607)

* build: unify YARN_VERSION variable usage and ensure CI uses yarn not npm

* chore: use a JS helper so that it can work on windows

* chore: make script/yarn without node_modules installed

script/yarn.js

@@ -0,0 +1,18 @@
+const cp = require('child_process')
+const fs = require('fs')
+const path = require('path')
+
+const YARN_VERSION = /'yarn_version': '(.+?)'/.exec(fs.readFileSync(path.resolve(__dirname, '../DEPS'), 'utf8'))[1]
+
+exports.YARN_VERSION = YARN_VERSION
+
+// If we are running "node script/yarn" run as the yarn CLI
+if (process.mainModule === module) {
+  const NPX_CMD = process.platform === 'win32' ? 'npx.cmd' : 'npx'
+
+  const child = cp.spawn(NPX_CMD, [`yarn@${YARN_VERSION}`, ...process.argv.slice(2)], {
+    stdio: 'inherit'
+  })
+
+  child.on('exit', code => process.exit(code))
+}