include vcov in the valgrind spkg

[rlmill]

http://www.nabble.com/Experimental-Valgrind-coverage-tool-(fwd)-td15321809.html

Component: packages: optional

Issue created by migration from https://trac.sagemath.org/ticket/4128

[orlitzky]

Obsolete:

commit 9961139f8bfc2d346ac52762602f12bc1a4f068e
Author: Matthias Koeppe <mkoeppe@math.ucdavis.edu>
Date:   Fri Jun 28 21:58:31 2024 -0700

    build/pkgs/valgrind: Reduce to dummy package