nix: build with GHC 9.12.3

Jumping past GHC 9.6, 9.8 and 9.10 immediately, because these versions
do not successfully build the static binary with TemplateHaskell and
iserv-proxy, yet.

Michal wrote the changes to hsie.

Co-authored-by: Michal Kleczek <michal@kleczek.org>

postgrest.cabal

@@ -16,8 +16,6 @@ build-type:         Simple
 extra-source-files: CHANGELOG.md
 
 tested-with:
-    -- nix
-    GHC == 9.4.8
     -- cabal on Ubuntu
   , GHC == 9.6.7
     -- cabal on Ubuntu
@@ -26,6 +24,7 @@ tested-with:
     -- stack on FreeBSD, MacOS, Ubuntu, Windows
   , GHC == 9.10.3
     -- cabal on Ubuntu
+    -- nix
   , GHC == 9.12.3
 
 source-repository head