Fix TERM env argument being misplaced in Flatpak
As it was previously, it only worked as intended if the user had only the binary as the start command (e.g. /bin/bash). Otherwise, it would had appended the flatpak-spawn related argument after the start command program.
Instead of passing the start command args to KSandbox::makeHostContext,
this will append them to the ones returned by makeHostContext on the
'postProcessArgs' function. In case Konsole is being run as a Flatpak,
all of said args would be added after --env=TERM=xterm-256color and
the program name.
An alternative would be to modify KSandbox::makeHostContext behaviour, but that would require more work to be done. With this MR I intended to fix the issue whilst making the least possible number of changes to the codebase.