feat: added env variable passing to labgrid_run

Added the ability to pass environment variables down into binaries being ran with labgrid_run.

There is a new test target in e2e/docker:

hello_world_test - a labgrid_genrule that uses labgrid_run and ape:printenv that prints out 'HELLO' env var that is provided as part of the cmd.

There are two main approaches to specifying environment variables:

• Providing --env MY_ENV=MY_VALUE in the cmd for labgrid_run.
• Specifying just the environment variable key and not the value like --env MY_ENV will attempt to resolve the value using the hosts environment.

For example using this target:

labgrid_run(
    name = "hello_world",
    srcs = ["@ape//ape:printenv"],
    cmd = "--env HELLO -- $(location @ape//ape:printenv) HELLO > $@",
    outs = ["hello_world.log"],
    platform = ":platform",
    tags = [
        "manual",
        "requires-docker",
    ],
)

you would need to specify the environment variable as part of the cli:

bazel build //docker:hello_world --action_env HELLO=MY_OTHER_VALUE

This would result in HELLO being set to 'MY_OTHER_VALUE'.