default PARAM isn't being injected during extension emulation

[russellwheatley]

[REQUIRED] Environment info

firebase-tools:
8.17.0

Platform:
macOS Catalina 10.15.4

[REQUIRED] Test case

See steps to reproduce.

[REQUIRED] Steps to reproduce

1. git clone git@github.com:firebase/extensions.git
2. run npm i from root of extension's directory.
3. cd delete-user-data directory
4. create test-params.env file. Add environment variables & insert project id as appropriate:

LOCATION=europe-west2
STORAGE_PATHS={DEFAULT}/test/{UID}-test.png
SELECTED_DATABASE_INSTANCE=[INSERT YOUR OWN PROJECT ID]
FIRESTORE_DELETE_MODE=shallow

5. run CLI command with your project id: firebase ext:dev:emulators:start --test-params=test-params.env --project=[INSERT YOUR OWN PROJECT ID]

[REQUIRED] Expected behavior

The auth emulator and the cloud function emulator running.

[REQUIRED] Actual behavior

The emulator command is erroring because the ${DATABASE_INSTANCE}' param which is auto populated in the extensions.yaml during build time, is not being recognised by the emulator.

Error: The extension.yaml has the following errors: 
  - Param SELECTED_DATABASE_INSTANCE has default value '${DATABASE_INSTANCE}', which does not pass the validationRegex ^([0-9a-z_.-]*)$

I did also try ${param:DATABASE_INSTANCE} in the extension.yaml but I got the same error:

Error: The extension.yaml has the following errors: 
  - Param SELECTED_DATABASE_INSTANCE has default value '${param:DATABASE_INSTANCE}', which does not pass the validationRegex ^([0-9a-z_.-]*)$