doc: Replace incorrect uses of @var with @code.

@var is for meta-syntactic variables but it is used in some places for
mere variables, which is incorrect and leads to inconsistent layout in
PDF and HTML.

* doc/guix.texi (package Reference, The Store Monad)
(G-Expressions, operating-system Reference)
(File Systems, Base Services, Log Rotation)
(Networking Services, Invoking guix deploy): Replace incorrect uses of
@var with @code.