[flake] fix libpq dependency

cabal.project

@@ -12,6 +12,10 @@ tests: True
 packages:
     ./
 
+-- https://github.com/input-output-hk/haskell.nix/issues/2281
+package postgresql-libpq
+  flags: +use-pkg-config
+    
 source-repository-package
     type: git
     location: https://github.com/AccelerateHS/accelerate.git

cabal.project.freeze

@@ -386,7 +386,7 @@ constraints: any.Boolean ==0.2.4,
              any.port-utils ==0.2.1.0,
              any.postgres-options ==0.2.2.0,
              any.postgresql-libpq ==0.10.2.0,
-             postgresql-libpq -use-pkg-config,
+             postgresql-libpq +use-pkg-config,
              any.postgresql-libpq-configure ==0.10.0.1,
              any.postgresql-simple ==0.7.0.0,
              any.pretty ==1.1.3.6,