[Buildsystem] Improve cache handling

Prevents cache issues by not purging cache before starting a build.
Splits cache purge related code from progress code and delays the purge
until after final build is done.

SConstruct

@@ -1092,7 +1092,6 @@ if "check_c_headers" in env:
             env.AppendUnique(CPPDEFINES=[headers[header]])
 
 
-# FIXME: This method mixes both cosmetic progress stuff and cache handling...
 methods.show_progress(env)
 # TODO: replace this with `env.Dump(format="json")`
 # once we start requiring SCons 4.0 as min version.
@@ -1124,3 +1123,5 @@ def purge_flaky_files():
 
 
 atexit.register(purge_flaky_files)
+
+methods.clean_cache(env)