UI: Copy result of getenv before use

Another call to getenv, as well as a call to the POSIX functions
setenv(), unsetenv(), and putenv() may invalidate the pointer
returned by a previous call or modify the string obtained from a
previous call.

https://en.cppreference.com/w/c/program/getenv
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f