build: use define_variable for get_pkgconfig_variable
On NixOS all packages are installed into their own immutable prefix. Because of this get_pkgconfig_variable will return a path from within systemd's prefix and we cannot write to it. By using define_variable we can replace the respective directory to be from the paths from meson.
https://github.com/NixOS/nixpkgs/pull/81384
Patch is on behalf of NixOS's nixpkgsEdited by worldofpeace