Makefile: Update to quay.io/benz0li/ghc-musl:9.8.4HEADmaster

https://gitlab.com/benz0li/ghc-musl lists four newer GHC versions
(9.8.4, 9.10.1, and 9.12.1).  With version 9.8.4, a test build of
git-annex 10.20250102 completed successfully.

For the other versions, dependency resolution fails.  Here's the
failure out for 9.10.1:

  Could not resolve dependencies:
  [__0] trying: git-annex-10.20250102 (user goal)
  [__1] trying: filepath-bytestring-1.5.2.0.0 (dependency of git-annex)
  [__2] next goal: base (dependency of git-annex)
  [__2] rejecting: base-4.20.0.0/installed-inplace (conflict:
  filepath-bytestring => base>=4 && <4.20)
  [__2] skipping: base; 4.21.0.0, 4.20.0.1, 4.20.0.0 (has the same
  characteristics that caused the previous version to fail: excluded
  by constraint '>=4 && <4.20' from 'filepath-bytestring')
  [__2] rejecting: base; 4.19.2.0, [...], 3.0.3.1 (constraint from
  non-reinstallable package requires installed instance)
  [__2] fail (backjumping, conflict set: base, filepath-bytestring, git-annex)
  After searching the rest of the dependency tree exhaustively, these
  were the goals I've had most trouble fulfilling: git-annex, base,
  filepath-bytestring

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