When running a command in the chroot, don't set TMP (and similar)
variables because those directories might nog make sense in the
target (yet).
FIXES#2269
The background idea is that, while CalamaresUtils::System::runCommand()
is a useful general API, it is
- still missing flexibility
- lacking a way to process output from the command "as it happens"
Waiting until the process ends, and then reading all stdout, is
inconvenient for processes that produce a **lot** of output,
and also makes it impossible to report progress. One module
in calamares-extensions has its own run-a-process implementation
for reading output, and this branch aims to introduce something
similar into Calamares core.
