Resolves #6746

The --env-file option added in PR #6535 makes sense as a top-level option but was added both to TopLevelCommand and to some subcommands. At best, the handling in the subcommands is redundant and muddies the waters; at worst, it causes bugs, like the fact that, as noted by @digitalist, docker-compose --env-file=custom.env up doesn't work, loading the .env file despite the attempt to override it.

So this removes the option from build, up, and down; moves the handling of the option into a method on TopLevelCommand; and changes the naming a bit to emphasize that this is a top-level option that controls the top-level environment (I've also been thinking of it as "project-level", as opposed to "service-level").

Testing

This does two main things:

• Removes the --env-file option from the build, up, and down subcommands. You can see that in their usage messages (which you can view by trying to put the option after the subcommand...)
• Resolves the issue whereby mishandling by some subcommands caused the top-level option to be ignored. For that, you can do something like this, which creates a simple docker-compose.yml and a few env files and runs a command to show that the override is happening (do it in a new directory to avoid clobbering anything important):

cat <<\EOF > docker-compose.yml
version: '3.7'
services:
  test:
    image: busybox
    env_file: .env.conf
    entrypoint: env
EOF

echo 'WHEREAMI=default' > .env
echo "This won't parse as a variable" >> .env
echo 'WHEREAMI' > .env.conf
echo 'DEFAULT_CONF_LOADED=true' >> .env.conf
echo 'WHEREAMI=override' > .env.override

docker-compose --env-file .env.override up

The output will include WHEREAMI=override when it's working. When it's not, i.e. on master, it'll produce an error saying there can't be whitespace in a variable name.

This doesn't add any cases to the test suite because I couldn't really think of good cases to test. PR #6535 added tests for the "working override" case and there were already tests for the "working default" case, and this mostly removes the stuff that's not covered by those. Adding a test that e.g. up works the same as config seems like it would take a bit of doing and wouldn't test anything very interesting under the circumstances.