nixos/nginx: fix test

When using format-strings, curly brackets need to be escaped using `{{`
to avoid errors from python.

And apparently, Perl's `==` is used to compare substrings[1] which is why
the translation to `assert http_code == "304"` failed as the string
contains several headers from curl.

[1] Just check `perl <(echo 'die "alarm" if "foo\n304" == 304')`