Remove state tracking, use KAS_WORK_DIR env variable

Config.has_changed didn't work reliably so far, so the state cache was
only allowing to associate a work directory with configuration file.
However this was rather unintuitive, specifically when moving the work
dir to a different home - nothing told the user about the persistent
association.

We therefore agreed to remove the state cache for now. It can be
reintroduced at any time once a consistent usage model exists.

To establish a relationship to a work directory inside a shell session,
kas now evaluates the KAS_WORK_DIR environment variable.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/build.py

@@ -67,7 +67,6 @@ class Build:
         macro.add(ReposCheckout())
         macro.add(SetupEnviron())
 
-        # TODO if cfg.has_changed() was not working properly
         macro.add(WriteConfig())
 
         # Build