Improve support for valgrind

[jayvdb]

https://formulae.brew.sh/formula/valgrind exists.

Also it looks like CI skips valgrind on the containers because snap is missing. IMO if snap is missing, using the system package for valgrind would be a good fallback.

It doesnt exist on choco yet, and https://sourceforge.net/p/valgrind4win/code/HEAD/tree/ looks dead.

[taiki-e]

https://formulae.brew.sh/formula/valgrind exists.

(It's x86_64 linux-only.)

IMO if snap is missing, using the system package for valgrind would be a good fallback.

That seems reasonable to me.

[jayvdb]

Just fyi, #526 doesnt include valgrind