Load project-specific environment variables in the Functions Emulator

[taeold]

Today, Functions Emulator does not load project-specific environment variables. This contradicts the public documentation which states:

When using a local Cloud Functions emulator, you can override environment variables for your project by setting up a .env.local file. Contents of .env.local take precedence over .env and the project-specific .env file.

https://firebase.google.com/docs/functions/config-env#emulator_support

The patch fixes the bug by making appropriate changes to load project-specific environment variables (either in .env.projectId or .env.projectAlias) when emulating functions.

Fixes #4239