Closes https://github.com/ziglang/zig/issues/19284
As of 9f2cb920c055bc990cc9d593b8dc9eaa450d07b9 configuring the global
cache directory to a relative path causes errors of the following form
when cross compiling to `windows-gnu`.
```
error: unable to generate DLL import .lib file for kernel32: FileNotFound
```
The issue occurred because the `def_final_path` included the global
cache path as a prefix and later on the `def_final_file` was created
using that path with the global cache directory handle.
dbb11915bd