environment: bugfix for set() and iter()

environment: bugfix for set() and iter()

Both problems behaved in a deterministic way and didn't produce
problematic EnvID changes

set() failed on duplicate vars, inserting the same var more than once
into the sorted table

iter() was using next(), which is not guaranteed to follow the table
in numerical order

Unit tests have been added for this issue.

Signed-off-by: Tobias Ulmer <tu@emlix.com>