Routes in .blitz dir broken when using multiple apps in a monorepo

### What is the problem?

Multiple apps in monorepo write to the same `.blitz` directory which is hoisted into the monorepo's top-level `node_modules`. The issue is also described here: https://github.com/blitz-js/legacy-framework/issues/92, including a temporary workaround.

### Paste all your error logs here:

No logs

### Paste all relevant code snippets here:

No snippets

### What are detailed steps to reproduce this?

1. Create two blitz apps in a monorepo and yarn workspaces hoist your dependencies into a top-level `node_modules`.
2. Run `blitz codegen`, inspect `/node_modules/.blitz` in one of the apps.
3. Run in the other.

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

```
macOS Big Sur | darwin-x64 | Node: v14.17.5

blitz: 0.41.1 (local)

Package manager: npm
System:
OS: macOS 11.5.2
CPU: (8) x64 Apple M1
Memory: 154.82 MB / 16.00 GB
Shell: 5.8 - /bin/zsh
Binaries:
Node: 14.17.5 - /var/folders/0t/qf92ltfx37jblv7t_3_7v8mh0000gn/T/yarn--1635001662121-0.45095447188894155/node
Yarn: 1.22.17 - /var/folders/0t/qf92ltfx37jblv7t_3_7v8mh0000gn/T/yarn--1635001662121-0.45095447188894155/yarn
npm: 6.14.15 - /var/folders/0t/qf92ltfx37jblv7t_3_7v8mh0000gn/T/fnm_multishells/46353_1634987771186/bin/npm
Watchman: Not Found
npmPackages:
@prisma/client: Not Found
blitz: 0.41.1 => 0.41.1
prisma: Not Found
react: Not Found
react-dom: Not Found
typescript: Not Found
```
Psst! the Not Found stuff happens when you use a monorepo, too.

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

_No response_