CI builds failing

[David Feuer]

It looks like the recent differential builds are all failing like this:

compiler/main/DynamicLoading.hs:79:19: warning: [-Wunused-matches]
    Defined but not used: ‘hsc_env’
   |
79 | initializePlugins hsc_env df
   |                   ^^^^^^^
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.1 for x86_64-unknown-linux):
	runtimeRepPrimRep
  typePrimRep (r_apc3 :: TYPE rep_apc2)
  rep_apc2
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/simplStg/RepType.hs:360:5 in ghc:RepType
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
make[1]: *** [compiler/stage1/build/PrelRules.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [all] Error 2


Oddly, I don't see a commit to master failing like this. What's going on?

David