Add AWS_CONFIG_FILE to the environment variables

Turns out there is an additional environment variable AWS_CONFIG_FILE,
which gets evaluated for finding configuration files as well. This
commit adds support for it.
diff --git a/pre_commit_hooks/detect_aws_credentials.py b/pre_commit_hooks/detect_aws_credentials.py
index 420333d..ed895d6 100644
--- a/pre_commit_hooks/detect_aws_credentials.py
+++ b/pre_commit_hooks/detect_aws_credentials.py
@@ -10,8 +10,8 @@
 def get_aws_credential_files_from_env():
     """Extract credential file paths from environment variables."""
     files = set()
-    for env_var in {'AWS_CREDENTIAL_FILE', 'AWS_SHARED_CREDENTIALS_FILE',
-                    'BOTO_CONFIG'}:
+    for env_var in {'AWS_CONFIG_FILE', 'AWS_CREDENTIAL_FILE',
+                    'AWS_SHARED_CREDENTIALS_FILE', 'BOTO_CONFIG'}:
         try:
             files.add(os.environ[env_var])
         except KeyError:
diff --git a/tests/detect_aws_credentials_test.py b/tests/detect_aws_credentials_test.py
index 410a33f..a366f01 100644
--- a/tests/detect_aws_credentials_test.py
+++ b/tests/detect_aws_credentials_test.py
@@ -19,8 +19,10 @@
     assert get_aws_credential_files_from_env() == {'/foo', '/bar'}
     monkeypatch.setenv('BOTO_CONFIG', '/baz')
     assert get_aws_credential_files_from_env() == {'/foo', '/bar', '/baz'}
+    monkeypatch.setenv('AWS_CONFIG_FILE', '/xxx')
+    assert get_aws_credential_files_from_env() == {'/foo', '/bar', '/baz', '/xxx'}
     monkeypatch.setenv('AWS_DUMMY_KEY', 'foobar')
-    assert get_aws_credential_files_from_env() == {'/foo', '/bar', '/baz'}
+    assert get_aws_credential_files_from_env() == {'/foo', '/bar', '/baz', '/xxx'}
 
 
 def test_get_aws_secrets_from_env(monkeypatch):