skip installation for SKIP'd hooks

pre_commit/commands/run.py

@@ -271,11 +271,11 @@ def _get_diff() -> bytes:
 def _run_hooks(
         config: Dict[str, Any],
         hooks: Sequence[Hook],
+        skips: Set[str],
         args: argparse.Namespace,
         environ: MutableMapping[str, str],
 ) -> int:
     """Actually run the hooks."""
-    skips = _get_skips(environ)
     cols = _compute_cols(hooks)
     classifier = Classifier.from_config(
         _all_filenames(args), config['files'], config['exclude'],
@@ -403,9 +403,11 @@ def run(
             )
             return 1
 
-        install_hook_envs(hooks, store)
+        skips = _get_skips(environ)
+        to_install = [hook for hook in hooks if hook.id not in skips]
+        install_hook_envs(to_install, store)
 
-        return _run_hooks(config, hooks, args, environ)
+        return _run_hooks(config, hooks, skips, args, environ)
 
     # https://github.com/python/mypy/issues/7726
     raise AssertionError('unreachable')

tests/commands/run_test.py

@@ -600,6 +600,29 @@ def test_skip_aliased_hook(cap_out, store, aliased_repo):
         assert printed.count(msg) == 1
 
 
+def test_skip_bypasses_installation(cap_out, store, repo_with_passing_hook):
+    config = {
+        'repo': 'local',
+        'hooks': [
+            {
+                'id': 'skipme',
+                'name': 'skipme',
+                'entry': 'skipme',
+                'language': 'python',
+                'additional_dependencies': ['/pre-commit-does-not-exist'],
+            },
+        ],
+    }
+    add_config_to_repo(repo_with_passing_hook, config)
+
+    ret, printed = _do_run(
+        cap_out, store, repo_with_passing_hook,
+        run_opts(all_files=True),
+        {'SKIP': 'skipme'},
+    )
+    assert ret == 0
+
+
 def test_hook_id_not_in_non_verbose_output(
         cap_out, store, repo_with_passing_hook,
 ):