fix: avoid changing os.environ in Harness (#1359)

Harness needs to provide a `JUJU_VERSION` value to set up the
`_JujuContext`, but it doesn't have to be in `os.environ`, it can just
be in the dictionary passed to create the `_JujuContext` object.

In production, `os.environ` would actually have this, so it's more
realistic for it to be present, but it would also have lots of the other
`JUJU_` environment variables as well, and we don't want to have Harness
simulate all those in the environment - we want people working with the
ops tools to access those, not the environment directly.

This change [broke the tests of at least one
charm](canonical/charm-simple-streams#22)
because it patches the environment to have specific values, and then
creating the `Harness` object changes that. It seems better for us to
not do this - if we did want to populate the environment to mimic Juju
then we'd likely want that to be explicit, or done around the event
emitting. It was also an accidental backwards compatiblity break.

ops/testing.py

@@ -260,9 +260,10 @@ def __init__(
         actions: Optional[YAMLStringOrFile] = None,
         config: Optional[YAMLStringOrFile] = None,
     ):
-        if 'JUJU_VERSION' not in os.environ:
-            os.environ['JUJU_VERSION'] = '0.0.0'
-        self._juju_context = _JujuContext.from_dict(os.environ)
+        context_environ = os.environ.copy()
+        if 'JUJU_VERSION' not in context_environ:
+            context_environ['JUJU_VERSION'] = '0.0.0'
+        self._juju_context = _JujuContext.from_dict(context_environ)
         self._charm_cls = charm_cls
         self._charm: Optional[CharmType] = None
         self._charm_dir = 'no-disk-path'  # this may be updated by _create_meta