Having lived in the Phoenix metro area for the last 27 years, and also spending ~115 nights at the bottom of the Canyon, I can tell you that 90 degrees is hot no matter what the humidity is. While it will be cooler in the shade at GCNP it will still be hot. Since most of the touristy things to do are in the open sun, Canyon overlooks, hiking and walking along the rim, you will not be in the shade much. I have a favorite saying when people start talking about how "comfortable" a dry heat is. It is this," Ovens are dry heat and no one sticks their head in there." As long as you are not doing long hikes or mule rides to Phantom you should not be overwhelmed by 90 degrees in July. It will be a very busy time and expect to see hundreds, if not thousands, of people there when you visit. Many of them from out of the country. Hope you have a great time.
RichH