Fix base-v.11.1 fix

[keleshev]

I had to replace ${null} with /dev/null for the fix to work. In my environment (Cygwin) ${null} expands to a local file called nul and later dune fails trying to delete this file. Using unix-ism like /dev/null is warranted here, I think, since it is already explicitly calls into bash.

This works for me, but I don't know if this will work for everybody. Presumably this worked before for some setups?

[fdopen]

It worked in the past. I don't know, why or since when it doesn't work any longer. But the fix is obviously correct. I've updated it in c41e34eb85e0c72ba2a2ef55aae702788e77b83a , your commit doesn't contain an update of the m5dsum.