Set format install to fix git repo builds

[nuclearkevin]

Description

When providing fmt (in a git repo) to OpenMC with FetchContent, a build failure is encountered with the following error message:

CMake Error: install(EXPORT "openmc-targets" ...) includes target "libopenmc_objects" which requires target "fmt" that is not in any export set.

This is caused by fmt not generating an install target when it is included as a subproject. Adding set(FMT_INSTALL ON) before fetching fmt fixes the build error.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @nuclearkevin!