implement support for yaml !Env tag

This way environment variables van be referenced in the
.pre-commit-config.yaml, e.g.

  cfg: !Env "${PRE_COMMIT_CONFIG_PATH}/.oelint-adv.yaml"

pre_commit/yaml.py

@@ -1,11 +1,26 @@
 from __future__ import annotations
 
 import functools
+import os
+import re
 from typing import Any
 
 import yaml
 
+
+def env_constructor(loader: yaml.Loader, node: yaml.ScalarNode) -> str:
+    """Load !Env tag"""
+    value = str(loader.construct_scalar(node))
+    match = re.compile('.*?\\${(\\w+)}.*?').findall(value)
+    if match:
+        for key in match:
+            value = value.replace(f'${{{key}}}', os.getenv(key, ''))
+    return value
+
+
 Loader = getattr(yaml, 'CSafeLoader', yaml.SafeLoader)
+Loader.add_constructor(tag='!Env', constructor=env_constructor)
+
 yaml_compose = functools.partial(yaml.compose, Loader=Loader)
 yaml_load = functools.partial(yaml.load, Loader=Loader)
 Dumper = getattr(yaml, 'CSafeDumper', yaml.SafeDumper)