Allow dotenv files from different environments for SSR and SSG in Next.js

[chalosalvador]

Description

This PR is intended to fix the issue #7207 by checking if the .env.<PROJECT-ID> file exists and make it available for the build process.

Scenarios Tested

1. Initialized a hosting project with Next.js
2. Created a .env.<PROJECT-ID> file in the root of the Next.js project
3. Created a page that uses SSR and accesses the environment variable in .env.<PROJECT-ID>
4. Created a page that uses SSG and accesses the environment variable in .env.<PROJECT-ID>
5. Deployed the project to Firebase hosting.
6. Make sure the environment variable in the .env.<PROJECT-ID> is available in a server side rendered page.
7. Make sure the environment variable in the .env.<PROJECT-ID> is available in a static generated page.

Sample Commands