Handle standalone pip creation from pip wheel

This change ensures that when pip is executed from a wheel/zip,
standalone pip creation for build environment reuses the source.

Resolves: pypa#9953

news/9953.bugfix.rst

@@ -0,0 +1 @@
+Fix PEP 517 builds when pip is executed from within a wheel.

src/pip/_internal/build_env.py

@@ -50,9 +50,9 @@ def _create_standalone_pip() -> Iterator[str]:
     """
     source = pathlib.Path(pip_location).resolve().parent
 
-    # Return the current instance if it is already a zip file. This can happen
-    # if a PEP 517 requirement is an sdist itself.
-    if not source.is_dir() and source.parent.name == "__env_pip__.zip":
+    # Return the current instance if source is not a directory. This can happen
+    # if a PEP 517 requirement is an sdist itself or when pip is already standalone.
+    if not source.is_dir():
         yield str(source)
         return