Use relative path for lib64 symlink (#4268)
## Summary
Closes https://github.com/astral-sh/uv/issues/4265.
## Test Plan
```
❯ ls -l .venv
total 16
-rw-r--r-- 1 crmarsh staff 43 Jun 12 09:23 CACHEDIR.TAG
drwxr-xr-x 14 crmarsh staff 448 Jun 12 09:23 bin
drwxr-xr-x 3 crmarsh staff 96 Jun 12 09:23 lib
lrwxr-xr-x 1 crmarsh staff 3 Jun 12 09:23 lib64 -> lib
-rw-r--r-- 1 crmarsh staff 174 Jun 12 09:23 pyvenv.cfg
```
```
❯ ls .venv/lib64/
python3.12
```
