Fix crash when no -e

`tox-envfile` has begun crashing when `tox` is run without any `-e`
argument:

```terminal
Traceback (most recent call last):
  ...
  File ".../.tox/.tox/lib/python3.8/site-packages/tox_envfile/main.py", line 13, in tox_configure
    for env in config.option.env:
TypeError: 'NoneType' object is not iterable
```

I have no idea what changed in `tox-envfile` to cause this to start
happening, but this commit attempts to fix the issue.

Author: seanh

src/tox_envfile/main.py

@@ -10,6 +10,6 @@
 def tox_configure(config):
     envvars = dotenv.dotenv_values(os.path.join(config.toxinidir, ".devdata.env"))
 
-    for env in config.option.env:
+    for envconfig in config.envconfigs.values():
         for name, value in envvars.items():
-            config.envconfigs[env].setenv[name] = value
+            envconfig.setenv[name] = value