don't record 'view: True' in environment config (#11182)

The default (implied) behavior for all environments, as of ea1de6b,
is that an environment will maintain a view in a location of its
choosing. ea1de6b explicitly recorded all three possible states of
maintaining a view:

 1. Maintain a view, and let the environment decide where to put it
    (default)
 2. Maintain a view, and let the user decide
 3. Don't maintain a view

This commit updates the config writer so that for case [1], nothing
will be written to the config.yaml. This will not change any existing
behavior, it just serves to keep the config more compact.

lib/spack/spack/environment.py

@@ -1029,7 +1029,14 @@ def write(self):
             view = self._view_path
         else:
             view = False
-        config_dict(self.yaml)['view'] = view
+
+        if view is not True:
+            # The default case is to keep an active view inside of the
+            # Spack environment directory. To avoid cluttering the config,
+            # we omit the setting in this case.
+            yaml_dict['view'] = view
+        elif 'view' in yaml_dict:
+            del yaml_dict['view']
 
         # if all that worked, write out the manifest file at the top level
         with fs.write_tmp_and_move(self.manifest_path) as f: