Fix line ending in custom build steps of vcxproj files

We must use Windows line endings in .vcxproj files to separate command
lines of custom build steps.

This amends commit f65cfadd.

Fixes: QTBUG-81553
Change-Id: I8d257f3846af7006df7f8d462b8f44efdce6a1fd
Reviewed-by: Miguel Costa <miguel.costa@qt.io>

qmake/generators/win32/msbuild_objectmodel.cpp

@@ -310,7 +310,7 @@ static QString commandLinesForOutput(QStringList commands)
         if (!commands.at(i).startsWith("rem", Qt::CaseInsensitive))
             commands.insert(i + 1, errchk);
     }
-    return commands.join('\n');
+    return commands.join("\r\n");
 }
 
 static QString unquote(const QString &value)