fix(ci): build each postgresql version separately

The `makeCheckHarness` function depends on all postgresql versions.

Author: jfroche

nix/checks.nix

@@ -286,9 +286,9 @@
               '';
         in
         {
-          psql_15 = makeCheckHarness self'.packages."psql_15/bin";
-          psql_17 = makeCheckHarness self'.packages."psql_17/bin";
-          psql_orioledb-17 = makeCheckHarness self'.packages."psql_orioledb-17/bin";
+          psql_15 = self'.packages."psql_15/bin";
+          psql_17 = self'.packages."psql_17/bin";
+          psql_orioledb-17 = self'.packages."psql_orioledb-17/bin";
           inherit (self'.packages)
             wal-g-2
             wal-g-3