Avoid spawning a shell when running node in test (#155)

`execSync` by default spawns a shell to exec the specified command. And,
without specifying an absolute path to the node binary, it will be
searched in `PATH` where yarn prepends a temporary directory containing
a shell script called **node** that wraps the original node binary with
exec, resulting in implicitly chaining a shell again.
On some Linux distros, the default non-interactive shell, such as the
Dash shell of Debian, silently discards dashes (`-`) in names of
environment variables. But GitHub Actions allows dashes in names of
input variables which are then converted to environment variables in the
form of `INPUT_<VARIABLE_NAME>`.
So the commit fix the error-prone test code by using execFileSync, which
by default exec directly without spawning a shell, to run node with its
absolute path.

__tests__/main.test.ts

@@ -19,9 +19,10 @@ test('wait 500 ms', async () => {
 // shows how the runner will run a javascript action with env / stdout protocol
 test('test runs', () => {
   process.env['INPUT_MILLISECONDS'] = '500'
+  const np = process.execPath
   const ip = path.join(__dirname, '..', 'lib', 'main.js')
-  const options: cp.ExecSyncOptions = {
+  const options: cp.ExecFileSyncOptions = {
     env: process.env
   }
-  console.log(cp.execSync(`node ${ip}`, options).toString())
+  console.log(cp.execFileSync(np, [ip], options).toString())
 })