[libcalamares] Close stdin on process jobs

- This avoids processes that wait on stdin, and e.g. improves
   reaction to having just "cat" (no file) in a command, or
   a package manager that asks for input.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -214,8 +214,8 @@ System::runCommand(
     if ( !stdInput.isEmpty() )
     {
         process.write( stdInput.toLocal8Bit() );
-        process.closeWriteChannel();
     }
+    process.closeWriteChannel();
 
     if ( !process.waitForFinished( timeoutSec ? ( timeoutSec * 1000 ) : -1 ) )
     {