Fix build with ghc 9.0.1

Continuing along the same lines as commit
2739adc258, it seems that
while Remote -> Retriever expands to the same data type this changes
it to, ghc 9.0.1 refuses to consider them equiviant. I guess it has
something to do with the forall?

The rest of the build all succeeds, although the stack build then crashes:
Linking .stack-work/dist/x86_64-linux-tinfo6/Cabal-3.4.0.0/build/git-annex/git-annex ...
Completed 233 action(s).
Prelude.chr: bad argument: 2214592520
This issue seems likely to be about it:
https://github.com/commercialhaskell/stack/pull/5508
I'm building with stack from debian, version 2.3.3, so a newer stack
probably avoids that. Anyway, despite that stack problem,
the git-annex binary is built, and works.

The stack.yaml I used for this build was patched as follows:

diff --git a/stack.yaml b/stack.yaml
index 8dac87c15..62c4b5b9d 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,6 +1,6 @@
 flags:
   git-annex:
-    production: true
+    production: false
     assistant: true
     pairing: true
     torrentparser: true
@@ -14,7 +14,7 @@ flags:
     httpclientrestricted: true
 packages:
 - '.'
-resolver: lts-18.13
+resolver: nightly-2021-09-07
 extra-deps:
 - IfElse-0.85
 - aws-0.22

Sponsored-by: Graham Spencer on Patreon

doc/bugs/Remote__47__Glacier.hs_build_error_GHC_9.0.1.mdwn

@@ -50,3 +50,4 @@ make: *** [Makefile:58: git-annex] Error 1
 ### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
 Oh absolutely. This version compiles fine under alpine 5.14, which uses GHC 8.8.4. I've been using this software for many years without major issues, and it's a linchpin of my backup infrastructure. Thanks for the all the good work!
 
+> [[fixed|done]] --[[Joey]]