[now-build-utils] Remove unused execa (#3427)

~~Reverts the execa bump from #3422~~

Removes `execa` since it is no longer used.

packages/now-build-utils/test/unit.test.js

@@ -1,9 +1,14 @@
 const path = require('path');
 const fs = require('fs-extra');
-const execa = require('execa');
 const assert = require('assert');
 const { createZip } = require('../dist/lambda');
-const { glob, download, detectBuilders, detectRoutes } = require('../');
+const {
+  glob,
+  download,
+  detectBuilders,
+  detectRoutes,
+  spawnAsync,
+} = require('../');
 const {
   getSupportedNodeVersion,
   defaultSelection,
@@ -39,7 +44,7 @@ it('should create zip files with symlinks properly', async () => {
   await fs.mkdirp(outDir);
 
   await fs.writeFile(outFile, await createZip(files));
-  await execa('unzip', [outFile], { cwd: outDir });
+  await spawnAsync('unzip', [outFile], { cwd: outDir });
 
   const [linkStat, aStat] = await Promise.all([
     fs.lstat(path.join(outDir, 'link.txt')),