[libcalamares] Remove TMP-related environment variables

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

CHANGES-3.3

@@ -29,6 +29,8 @@ This release contains contributions from (alphabetically by first name):
  - In CMake, "view" is no longer accepted as an alias of the module
    type "viewmodule" in function `calamares_add_plugin()`.
  - Plain Ubuntu builds have been added to the CI roster. (thanks Simon)
+ - Commands that run in the target system (in the chroot) no longer
+   use the TMP-related environment variables from the host. #2269
 
 ## Modules ##
  - The *displaymanager* module configuration for `greetd` has some more

src/libcalamares/utils/Runner.cpp

@@ -128,6 +128,14 @@ Calamares::Utils::Runner::run()
     {
         auto env = QProcessEnvironment::systemEnvironment();
         env.insert( "LC_ALL", "C" );
+        // No guarantees that host settings for /tmp/ make sense in target
+        if ( m_location == RunLocation::RunInTarget )
+        {
+            env.remove( "TEMP" );
+            env.remove( "TEMPDIR" );
+            env.remove( "TMP" );
+            env.remove( "TMPDIR" );
+        }
         process.setProcessEnvironment( env );
     }
     process.setProcessChannelMode( QProcess::MergedChannels );