Load multiple env files

### What is the problem?

I saw in the document, we can load multiple env files like this:

`blitz dev --env staging,production`

But when I try in my nextjs 13 project, it didn't work.

### Run `blitz -v` and paste the output here:

```
Blitz version: 2.0.0-beta.33 (global)
Blitz version: 1.0.0 (local)
macOS Ventura | darwin-arm64 | Node: v18.15.0

Package manager: npm

System:
OS: macOS 13.5
CPU: (8) arm64 Apple M1
Memory: 111.50 MB / 16.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 18.15.0 - ~/.nvm/versions/node/v18.15.0/bin/node
Yarn: 1.22.19 - ~/.nvm/versions/node/v18.15.0/bin/yarn
npm: 9.6.3 - ~/.nvm/versions/node/v18.15.0/bin/npm
npmPackages:
@blitzjs/auth: Not Found
@blitzjs/next: Not Found
@blitzjs/rpc: Not Found
@prisma/client: Not Found
blitz: Not Found
next: 13.4.2 => 13.4.2
prisma: Not Found
react: 18.2.0 => 18.2.0
react-dom: 18.2.0 => 18.2.0
typescript: 5.0.4 => 5.0.4
```

Start by reproducing `blitz dev --env staging,production` in the reported Next.js 13 environment, using the documented multiple-env-file behavior and the supplied Blitz and Node versions. Trace the command's environment-file loading entry point and compare the observed variables with the documented expectation; done means the behavior is resolved or the documentation accurately reflects the supported behavior.

Written by the indexing model from the issue text.