feat: Make db-uri optional

The default is now "postgresql://" which falls back to LIBPQ environment variables.

Resolves #1991, Ref #1823

nix/tools/loadtest.nix

@@ -45,6 +45,9 @@ let
         inRootDir = true;
       }
       ''
+        # previously required settings to make this work with older branches
+        export PGRST_DB_URI="postgresql://"
+
         export PGRST_DB_CONFIG="false"
         export PGRST_DB_POOL="1"
         export PGRST_DB_TX_END="rollback-allow-override"

nix/tools/withTools.nix

@@ -35,7 +35,7 @@ let
       }
       ''
         # avoid starting multiple layers of withTmpDb
-        if test -v PGRST_DB_URI; then
+        if test -v PGHOST; then
           exec "$_arg_command" "''${_arg_leftovers[@]}"
         fi
 
@@ -53,7 +53,6 @@ let
         export PGHOST="$tmpdir/socket"
         export PGUSER
         export PGDATABASE
-        export PGRST_DB_URI="postgresql:///$PGDATABASE?host=$PGHOST&user=$PGUSER"
         export PGRST_DB_SCHEMAS
         export PGRST_DB_ANON_ROLE