I am not aware of inscriptions in the Giza tombs, specifically mentioning rain or Nile flooding.
However, the heb-sed festival which is connected to pyramids is associated with rainfall and the flooding of the Nile.
Petrie provides an in-depth discussion on the heb-sed festival and its connection with rain and Nile flooding in Ancient Egypt, vol. 10. pp. 65-70. See the reference below:

[archive.org]