Use explicit path for vsce

extensions/ql-vscode/gulpfile.ts/package.ts

@@ -9,7 +9,7 @@ export async function packageExtension(): Promise<void> {
     'package',
     '--out', path.resolve(deployedPackage.distPath, '..', `${deployedPackage.name}-${deployedPackage.version}.vsix`)
   ];
-  const proc = childProcess.spawn('npx', ['vsce'].concat(args), {
+  const proc = childProcess.spawn('./node_modules/.bin/vsce', args, {
     cwd: deployedPackage.distPath
   });
   proc.childProcess.stdout!.on('data', (data) => {