This fixes the patch for nginx to clear the Last-Modified header if a
static file is served from the Nix store.
So far we only used the ETag from the store path, but if the
Last-Modified header is always set to "Thu, 01 Jan 1970 00:00:01 GMT",
Firefox and Chrome/Chromium seem to ignore the ETag and simply use the
cached content instead of revalidating.
Alongside the fix, this also adds a dedicated NixOS VM test, which uses
WebDriver and Firefox to check whether the content is actually served
from the browser's cache and to have a more real-world test case.
845e92835d