nix: move withtmpdb to separate file

Co-authored-by: Remo Rechkemmer <59358383+monacoremo@users.noreply.github.com>

nix/withtmpdb.nix

@@ -0,0 +1,20 @@
+# Wrap the `test/with_tmp_db` script with the required dependencies from Nix.
+{ checkedShellScript }:
+
+postgresql:
+checkedShellScript
+{
+  name = "postgrest-test-withtmpdb-${postgresql.name}";
+  docs = "Run the given command in a temporary database";
+  inRootDir = true;
+}
+  ''
+    # avoid starting multiple layers of with_tmp_db
+    if test ! -v PGRST_DB_URI; then
+      export PATH=${postgresql}/bin:"$PATH"
+
+      exec test/with_tmp_db "$@"
+    else
+      "$@"
+    fi
+  ''