Added `saltenv` support to `salt.modules.apt`.

Removed unnecessary intermediary variables. Minor formatting changes.

Thanks for the pointers @hvnsweeting!
Reference the current salt environment. Thanks @hvnsweeting!
Add missing `saltenv` support. Thanks @hvnsweeting.
Add missing `saltenv` support. Thanks @hvnsweeting!
Add missing `saltenv` support. Thanks @hvnsweeting!
Merge pull request #8320 from s0undt3ch/issues/8196-environments

Some more `saltenv` related changes/fixes