Do not switch to the original buffer.  Use `buffer-local-value`
instead.  The buffer switching is more costly, which slows down
continuously updating completion UIs like Icomplete or Vertico if
there are many variables.

Ideally there would be a `buffer-local-boundp` predicate we could use
here.

In GNU Emacs 28.0.50 (build 12, x86_64-pc-linux-gnu, GTK+ Version
3.24.5, cairo version 1.16.0)
 of 2021-05-23 built on projects
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)