[kterminallauncherjob] Inherit default process environment from parent

Before Qt 6.3, an empty (default constructed) QProcessEnvironment caused
QProcess to incorrectly inherit the environment variables from the parent
process. This was fixed in qtbase 5fc9c02a695.

CommandLauncherJob was adjusted for this in 91698494,
however here we are overriding its default env with our own default-constructed one

BUG: 482107