fix: check pip module availability instead of PATH binary in download (#13947)

_get_pip_install_cmd() uses shutil.which("pip") to check for pip, then
returns [sys.executable, "-m", "pip", "install"] which invokes pip as a
module. The binary check fails in venvs where ensurepip creates pip3 but
not pip (common on Debian/Ubuntu), even though python -m pip works.

Replace shutil.which("pip") with importlib.util.find_spec("pip"), which
tests the actual precondition: whether pip is importable in the current
interpreter. The uv branch is unchanged since uv is a standalone binary.

Fixes #13946

Author: taylorsatula

spacy/cli/download.py

@@ -1,3 +1,4 @@
+import importlib.util
 import shutil
 import sys
 from typing import Optional, Sequence
@@ -189,7 +190,7 @@ def download_model(
 
 
 def _get_pip_install_cmd() -> list:
-    if shutil.which("pip"):
+    if importlib.util.find_spec("pip") is not None:
         return [sys.executable, "-m", "pip", "install"]
     elif shutil.which("uv"):
         return ["uv", "pip", "install"]