I just remembered. Adding a -f <file> option to the GNU Coreutils env utility has previously been discussed:

https://lists.gnu.org/archive/html/coreutils/2021-10/msg0000...

It came up in the mailing also this March. I saw the posting in my inbox and proposed that a null-terminated format be handled, which is exactly like /proc/<pid>/env:

https://lists.gnu.org/archive/html/coreutils/2024-03/msg0014...

If that feature were available, this becomes

  env -f .env command arg ...