haskell demangler doesn't build

[mattgodbolt]

[1 of 1] Compiling Main             ( demangle.hs, demangle.o )

demangle.hs:1:1: error:
    Could not find module ‘Encoding’
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
1 | import Encoding
  | ^^^^^^^^^^^^^^^
make[1]: *** [Makefile:3: demangle] Error 1
make[1]: Leaving directory '/home/mgodbolt/dev/personal/ce/tools/haskell'
make: *** [Makefile:20: haskell-support] Error 2

Might be something in the update to 9.* or else it might be something I broke in the port to GH Actions, though it works with the older haskell (CC @RubenRBS )