env_file: fix broken test cases

TestWriteEnvFileDos2Unix had a copy/paste bug, it shouldn't have
asserted that mtime doesn't change because the file is actually being
modified in this test. This didn't come up earlier because the actual
comparison wasn't using Time.Equal as it should have.

Instead switch to comparing inode numbers which is the actual thing I
wanted to test for in the first place, just accessing them is much more
awkard. Now all tests where it is relevant check the inode in addition
to the contents.