tests/common/util.rs: add cfg(feature = "env")

[cakebaker]

The recent merge of #5869 has an unwanted side effect: running the tests for a single tool shows some failures unrelated to the tool:

$ cargo test --features "nl" --no-default-features

[snipped]

failures:

---- common::util::tests::test_simulation_of_terminal_false stdout ----
run: /home/dho/projects/coreutils/target/debug/coreutils env sh is_atty.sh
thread 'common::util::tests::test_simulation_of_terminal_false' panicked at tests/common/util.rs:3590:65:
Command was expected to succeed. Exit code: 1.
stdout = env: function/utility not found

 stderr = 

[snipped]

failures:
    common::util::tests::test_simulation_of_terminal_false
    common::util::tests::test_simulation_of_terminal_pty_pipes_into_data_and_sends_eot_automatically
    common::util::tests::test_simulation_of_terminal_pty_sends_eot_automatically
    common::util::tests::test_simulation_of_terminal_pty_write_in_data_and_sends_eot_automatically
    common::util::tests::test_simulation_of_terminal_size_information
    common::util::tests::test_simulation_of_terminal_true

This PR fixes the issue by adding #[cfg(feature = "env")] to the failing tests.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/mv/backup-dir is no longer failing!

[tertsdiepraam]

We really need that CI with the individual utils, don't we...