fix: replacing --cmd with && in custom build commands

[devLeopar]

This should resolve #103

here is some more information about this

This _generated folder never be created with the --cmd option. This is not a file path issue. I've tried combinations on next build however, the issue is the folder absence. It is replicated locally as well. You can use your CONVEX_DEPLOY_KEY and run that command to see

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[thomasballinger]

It sounds like you've got a custom setup where you don't commit the _generated file, generally it's recommended that already exist in version control.

The point of this --cmd is to avoid deploying the convex functions if the frontend build fails:

1. set .env.local environment variable
2. run frontend build
3. if that succeeded, build and deploy convex code.

We could change this a bit for setups like yours that dont' commit _generated by inserting a step between 1 and 2 that runs codegen, but the suggestion in the PR to unconditionally deploy convex functions isn't workable, we don't want to end up deploying new convex functions without deploying a new frontend because the frontend build is broken.