Assuming your .env file follows the format required by GITHUB_ENV you can just append it all:

steps:
  - name: read environment file
    run: cat .env >>${GITHUB_ENV}

If the .env file uses a different format you’ll have to run it through some kind of converter tool instead of simply copying, e.g. a script that reads the environment file and writes to GITHUB_ENV in the expected format.

Sorry, I somehow didn’t put the key word here, at the job level I mean. You can’t run anything at that level, right?