Don't warn when overriding environment variable with identical value

[gerritholl]

In my ATOVS_ENV8 I have DIR_DATA_TLE set to /data/pytroll/IN/TLE. In aapp-processing.yaml I have in_tle set to the same. Yet there is a warning logged to stderr:

[WARNING: 2021-01-21 08:44:42 : aapp_runner.tle_satpos_prepare] Override the env variable set in AAPP_ENV DIR_DATA_TLE from /data/pytroll/IN/TLE to /data/pytroll/IN/TLE.

This warning is not useful. Although the warning doesn't harm further processing, warnings that aren't useful are still noise that makes other warnings less prominently visible. It would be better if aapp-runner did not warn that it is changing the value of an environment variable when it's not true.