Test external commands with nightly GHC

[alt-romes]

Fixes #231

[alt-romes]

I think this will have to wait a bit further while head.hackage catches up:

 9 of 11] Compiling GHC.TypeNats.Experimental ( src/GHC/TypeNats/Experimental.hs, dist/build/GHC/TypeNats/Experimental.o, dist/build/GHC/TypeNats/Experimental.dyn_o )
src/GHC/TypeNats/Experimental.hs:15:1: error: [GHC-87110]
Error:     Could not load module ‘GHC.Num.Natural’.
    It is a member of the hidden package ‘ghc-internal-10.100.0’.
    Perhaps you need to add ‘ghc-internal’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.
   |
15 | import GHC.Num.Natural (naturalLog2)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[10 of 11] Compiling Prelude.Experimental ( src/Prelude/Experimental.hs, dist/build/Prelude/Experimental.o, dist/build/Prelude/Experimental.dyn_o )
[11 of 11] Compiling System.Mem.Experimental ( src/System/Mem/Experimental.hs, dist/build/System/Mem/Experimental.o, dist/build/System/Mem/Experimental.dyn_o )