MSVC module overwrites build environment

From windows-msvc.qbs:

    setupBuildEnvironment: {
        for (var key in buildEnv) {
            var v = new ModUtils.EnvironmentVariable(key);
            v.value = buildEnv[key];
            v.set();
        }
    }

As a result, all "appendable" variables such as PATH now contain only the values retrieved by the probe, while the original values are overwritten. These variables need to be merged instead.