sandbox: Remove instances of get_directory

sandbox/_mount.py, sandbox/_sandboxbwrap.py:
  Remove instances of get_directory

buildstream/sandbox/_mount.py

@@ -32,7 +32,8 @@ from .._fuse import SafeHardlinks
 class Mount():
     def __init__(self, sandbox, mount_point, safe_hardlinks):
         scratch_directory = sandbox._get_scratch_directory()
-        root_directory = sandbox.get_directory()
+        # Getting external_directory here is acceptable as we're part of the sandbox code.
+        root_directory = sandbox.get_virtual_directory().external_directory
 
         self.mount_point = mount_point
         self.safe_hardlinks = safe_hardlinks

buildstream/sandbox/_sandboxbwrap.py

@@ -56,7 +56,9 @@ class SandboxBwrap(Sandbox):
 
     def run(self, command, flags, *, cwd=None, env=None):
         stdout, stderr = self._get_output()
-        root_directory = self.get_directory()
+
+        # Allowable access to underlying storage as we're part of the sandbox
+        root_directory = self.get_virtual_directory().external_directory
 
         # Fallback to the sandbox default settings for
         # the cwd and env.