Create extension directory if not found

[jsjoeio]

Occasionally in CI or locally, we will see this error produced by code-server:

ERR /home/runner/.local/share/code-server/extensions [Error: ENOENT: no such file or directory, scandir '/home/runner/.local/share/code-server/extensions'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'scandir',
  path: '/home/runner/.local/share/code-server/extensions'
}

@code-asher says we can create environmentService.globalStorageHome and could add the ext dir there to prevent this error. Similar to how GitPod does it: https://github.com/gitpod-io/vscode/blob/gp-code/src/vs/gitpod/node/server.ts#L330