refactor(runner/entrypoint): check for externalstmp

[lovesegfault]

This fixes these annoying messages in the runner logs:

# Authentication
√ Connected to GitHub
# Runner Registration
√ Runner successfully added
√ Runner connection is good
# Runner settings
√ Settings Saved.
Runner successfully configured.
{
  "agentId": 123,
  "agentName": "runner-deployment-123",
  "poolId": 1,
  "poolName": "Default",
  "ephemeral": true,
  "serverUrl": "https://pipelines.actions.githubusercontent.com/123",
  "gitHubUrl": "https://github.com/123",
  "workFolder": "/runner/_work"
mkdir: cannot create directory './externals': File exists
mv: cannot stat './externalstmp/*': No such file or directory
}
√ Connected to GitHub

Current runner version: '2.289.1'
2022-03-27 11:04:41Z: Listening for Jobs

Emphasis:

mkdir: cannot create directory './externals': File exists
mv: cannot stat './externalstmp/*': No such file or directory

[toast-gear]

LGTM, cheers. I've removed the directory check as it shouldn't be needed and I'm hesitant changing the move logic in case it impacts some edge case