Probably a caching issue. Generally closing the browser and or deleting cache fixes the issue. It has sprung up before and most likely will be fixed by final release.

I'd also like to say thank you for the solution here. Since I knew install.php no longer existed, I figured the redirect must have been hard coded. I soon found the redirect in main.php but it appeared to be because there was no database connection. I checked and re-checked that and I didn't understand why ?flush=1 and asking for other pages worked, but not the home page. I ended up finding this forum post about page 4 of a search for install.php. Just to make it easier for someone else who may need this post I will add some other terms here that will come up on searching the forums.

Redirect to the installer if no database is selected.
Redirect to install.php
database connection
main.php redirects to install.php