CSRFTokenMismatchError when importing Prisma client in blitz.config.ts

### What is the problem?

When importing the `@prisma/client` package in `blitz.config.ts` (or when importing any file in `blitz.config.ts` that itself recursively references an import to `@prisma/client` anywhere in its dependency tree), any incoming request to Blitz fails with a `CSRFTokenMismatchError` in production mode.

For example, simply cloning and running the "auth" example from the current `canary` branch examples folder you get the exact same error when simply running `yarn build && yarn start`.

### Paste all your error logs here:

```
2021-08-03 09:37:59.205 ERROR Error while processing the request

CSRFTokenMismatchError
details:
{
name: 'CSRFTokenMismatchError',
statusCode: 401
}
```

### Paste all relevant code snippets here:

Code taken from `examples/auth/blitz.config.ts`

```typescript
import {sessionMiddleware, simpleRolesIsAuthorized} from "blitz"
import db from "db"
const withBundleAnalyzer = require("@next/bundle-analyzer")({
enabled: process.env.ANALYZE === "true",
})

module.exports = withBundleAnalyzer({
middleware: [
sessionMiddleware({
cookiePrefix: "blitz-auth-example",
isAuthorized: simpleRolesIsAuthorized,
getSession: (handle) =>
db.session.findFirst({where: {handle}}).then((session) => {
console.log(session)
return session
}),
}),
]
})

```

### What are detailed steps to reproduce this?

1. Clone the "auth" example locally and install dependencies
2. Run `yarn prisma migrate dev`
3. Run `yarn build && yarn start`
4. Go to `localhost:3000` (just to make sure: Do that in incognito mode, so that all cookies are cleared)
5. The server fails with a CSRFTokenMismatchError

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

Output from running `blitz -v` in the auth example folder
```
macOS Big Sur | darwin-x64 | Node: v16.5.0

blitz: 0.31.0 (global)
blitz: 0.39.0-canary.0 (local)

Package manager: yarn
System:
OS: macOS 11.5
CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
Memory: 760.63 MB / 16.00 GB
Shell: 5.8 - /usr/local/bin/zsh
Binaries:
Node: 16.5.0 - /usr/local/bin/node
Yarn: 1.22.10 - /usr/local/bin/yarn
npm: 7.19.1 - /usr/local/bin/npm
Watchman: 4.9.0 - /usr/local/bin/watchman
npmPackages:
@prisma/client: 2.24.1 => 2.24.1
blitz: 0.39.0-canary.0 => 0.39.0-canary.0
prisma: 2.24.1 => 2.24.1
react: 0.0.0-experimental-6a589ad71 => 0.0.0-experimental-6a589ad71
react-dom: 0.0.0-experimental-6a589ad71 => 0.0.0-experimental-6a589ad71
typescript: 4.3.5 => 4.3.5
```

### Please include below any other applicable logs and screenshots that show your problem:

_No response_