postgresqlPackages: use makeFlags USE_PGXS=1 consistently

pkgs/servers/sql/postgresql/ext/pg_similarity.nix

@@ -12,7 +12,9 @@ stdenv.mkDerivation {
   };
 
   buildInputs = [ postgresql gcc ];
-  buildPhase = "USE_PGXS=1 make";
+
+  makeFlags = [ "USE_PGXS=1" ];
+
   installPhase = ''
     install -D pg_similarity${postgresql.dlSuffix} -t $out/lib/
     install -D ./{pg_similarity--unpackaged--1.0.sql,pg_similarity--1.0.sql,pg_similarity.control} -t $out/share/postgresql/extension

pkgs/servers/sql/postgresql/ext/plr.nix

@@ -13,9 +13,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ R postgresql ];
-  preBuild = ''
-    export USE_PGXS=1
-  '';
+
+  makeFlags = [ "USE_PGXS=1" ];
+
   installPhase = ''
     install -D plr${postgresql.dlSuffix} -t $out/lib/
     install -D {plr--*.sql,plr.control} -t $out/share/postgresql/extension