🚸 Set PORTAGE_CONFIGROOT to / by default

In case gcm uses portage installed in a virtual environment,
portage looks for configuration files where it's installed.
This behavior might be confusing, so it's better to make
portage use configuration files from the root by default.

For a better understanding, see the descriptions of the
EPREFIX and PORTAGE_CONFIGROOT environment variables:
  - `man emerge`
  - https://dev.gentoo.org/~zmedico/portage/doc/man/emerge.1.html

src/gcm/__init__.py

@@ -1,2 +1,7 @@
+import os
+
+os.environ.setdefault('PORTAGE_CONFIGROOT', '/')
+
+
 __name__ = 'Gentoo Cache Manager'
 __version__ = '0.1.0'

tests/test_init.py

@@ -0,0 +1,16 @@
+import os
+import sys
+from unittest.mock import patch
+
+
+@patch.dict(os.environ)
+@patch.dict(sys.modules)
+def test_init():
+    assert os.environ['PORTAGE_CONFIGROOT'] == 'tests/data'
+
+    del os.environ['PORTAGE_CONFIGROOT']
+    sys.modules.pop('gcm', None)
+
+    __import__('gcm')
+
+    assert os.environ['PORTAGE_CONFIGROOT'] == '/'