@actions/exec doesn't support multiple commands

Describe the bug

Hello! I ran this script through @actions/exec:

await exec.exec('npm ci && npm run build --if-present')

It was expanded and run:

So it looks like the first npm command was prefixed with /usr/local/bin, but the second wasn't. It also didn't throw an error, which made this difficult to debug. What's the right way to

cc https://github.com/JasonEtco/build-and-tag-action/issues/4

To Reproduce

Create an action:

package.json:

{
  "scripts": {
    "build": "echo \"Hello!\""
  }
}

The action:

const exec = require('@actions/exec')

async function main () {
  await exec.exec('npm ci && npm run build')
}

main()

Observe that the build command isn't properly run.

Expected behavior

I'd expect the above command to "just work" - but if not, it should throw an error instead of fail silently.

Start with the @actions/exec package entry point and run the provided action using npm ci && npm run build, comparing the observed command expansion with the expected behavior. Trace how the command string is parsed and executed, then verify that multiple commands either run correctly or produce a clear error.