_service/pypi.py: Do not execute pip cache cmd if custom dir is given

In a scenario where the pip cache directory isn't available because the
user doesn't have permissions, pip will error with this message:

```
ERROR: pip cache commands can not function since cache is disabled.
```

This currently cannot be mitigated by specifying the `--cache-dir` flag
because the `python -m pip cache dir` command is execute before this
predicate is evaluated. By using an assignment expression (PEP 572), we
can evaluate whether the expression evaluates to a truthy value in the
`elif` branch (saving it's return value) AFTER first evaluating the
custom directory value.

pip_audit/_service/pypi.py

@@ -109,12 +109,11 @@ def _get_pip_cache() -> str:
 
 
 def _get_cache_dir(custom_cache_dir: Optional[Path]) -> str:
-    pip_cache_dir: Optional[str] = (
-        _get_pip_cache() if _PIP_VERSION >= _MINIMUM_PIP_VERSION else None
-    )
     if custom_cache_dir is not None:
         return str(custom_cache_dir)
-    elif pip_cache_dir is not None:  # pragma: no cover
+    elif (
+        pip_cache_dir := _get_pip_cache() if _PIP_VERSION >= _MINIMUM_PIP_VERSION else None
+    ) is not None:  # pragma: no cover
         return pip_cache_dir
     else:
         fallback_path = os.path.join(Path.home(), ".pip-audit-cache")