If you have the Epic Games Launcher installed in a drive other than C:\ you can create a symbolic directory link in C:\Program Files (x86)\Epic Games that points to the drive/directory you have the launcher installed. I used powershell to do this (do a web search for New-SymLink, I'm sure there are a few out there but I grabbed the function from the Powershell Gallery site - copy it to PS ISE, run once to load in memory then function will be usable) though you could just as easily use the command line built into windows, mklink. This in essence "tricks" the Epic installers into thinking your launcher is installed where they seem to expect it while simply redirecting the I/O to the actual install location. Symbolic links do not take up any space so this will not cause any additional consumption of space on your C: drive.
The reason I opted for this as opposed to other suggested solutions/workarounds is that this isn't the first time I've encountered a quirk with one of their installers with an odd affinity to drive C:. This way should the Epic Launcher programmers make a mistake like this again in the future then you are already covered.
(in my case Epic Games Launcher is installed on drive E:) - do this with Powershell or Command Prompt run as admin, otherwise unless you have disabled User Account Control neither will have enough privileges to create symbolic links.
Powershell syntax :
New-SymLink -Path "E:\Program Files (x86)\Epic Games" -SymName "C:\Program Files (x86)\Epic Games" -Directory
Command Line syntax:
mklink /D "C:\Program Files (x86)\Epic Games" "E:\Program Files (x86)\Epic Game"
In the future should you decide to remove the symbolic link for any reason you can delete it from explorer like you would any other shortcut (which is how this will show up); it will not delete any content from the target (actual) installation path.