env-file: cleanups for write_env_file_label()

env-file: cleanups for write_env_file_label()

Explicitly call label_ops_post() where necessary with specific
arguments. Previously, regardless if fopen_tmpfile_linkable_at()
success, label_ops_post() was called but its argument was heavily
conditionalized. Let's call it both on failure and success cases
with specific arguments. This should be easy to read.

This also
- moves variable declarations where used,
- drops unnecessary boolean flag call_label_ops_post.

No functional change. Just reafactoring.

Hopefully silence CID#1664328, though it is false-positive.