fix: rm only the content of `exportPath`

[m-shaka]

Description

I was having trouble with exporting emulator data when I ran an emulator with Docker. It was really hard to find the reason, but I finally found it:

• Emulator tries to remove the export path recursively.
  • docker volumes can not be removed, but rimraf doesn't throw any error (This is the issue of rimraf@3. The latest version raises an error as you expect)
• then , emulator tries to move the temp data directory to the export path that still exists, so dest already exists is raised.

Then, the change I made is here:

• Remove the content of the export path, instead of the export path itself
• Then copy the temp directory to the export path
• Remove the temp directory

reproduction

I prepared a different branch for reproduction. You can get the error below with docker compose up --build https://github.com/m-shaka/firebase-tools/tree/docker-repro

firebase_emulator-1  | i  auth: Importing config from /firebase-export/auth_export/config.json
firebase_emulator-1  | i  auth: Importing accounts from /firebase-export/auth_export/accounts.json
firebase_emulator-1  | i  Running script: 'exit 1'
firebase_emulator-1  | /bin/sh: exit 1: not found
firebase_emulator-1  | ⚠  Script exited unsuccessfully (code 127)
firebase_emulator-1  | i  Automatically exporting data using --export-on-exit "/firebase-export" please wait for the export to finish...
firebase_emulator-1  | i  Found running emulator hub for project test at http://127.0.0.1:4400
firebase_emulator-1  | i  Exporting data to: /firebase-export
firebase_emulator-1  | i  emulators: Received export request. Exporting data to /firebase-export.
firebase_emulator-1  | ⚠  emulators: Export failed: dest already exists.

Scenarios Tested

• export data on MacOS (v14.1.2)
• export data on Docker container (Docker for Mac version: 20.10.12)

I'm sorry that I didn't test on Windows

Sample Commands

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