Update nokogiri gem.

Fix gem version conflicts described in 20abd8a5f. Nokogiri couldn't be
upgraded past 1.10.9 because 1.11.0 causes a build failure in Nokogumbo
2.0.2, but we couldn't stay on 1.10.9 either because it has a hard
requirement on Ruby <2.7 and we require Ruby >=2.7. This made `bundle
update` fail with a Gemfile conflict.

The fix is to disable libxml2 support when building Nokogumbo. Nokogumbo
wants to use the same version of libxml2 as Nokogiri, but Nokogiri
1.11.0 changed how it reports which version of libxml2 it's using, which
causes Nokogumbo's build to fail. Disabling libxml2 may reduce
performance of Nokogumbo ([1]).

While we're at it, we also make Nokogiri use the system version of
libxml2 instead of its own bundled version. Nokogiri really wants
us to use its own patched version of libxml2 instead of the system
version, but the patches it applies look relatively minor and don't seem
relevant to us ([2]). Using the system version reduces build time during CI.

This adds libxml2 and libxslt as OS-level dependencies of Danbooru. You
may need to do `sudo apt-get install libxml2-dev libxslt-dev` to install
these libraries after this commit.

[1]: https://github.com/rubys/nokogumbo#flavors-of-nokogumbo
[2]: https://github.com/sparklemotion/nokogiri/tree/master/patches/libxml2

.github/workflows/test.yaml

@@ -64,7 +64,7 @@ jobs:
     - name: Install OS dependencies
       run: |
         apt-get update
-        apt-get -y install --no-install-recommends build-essential ruby ruby-dev ruby-bundler git nodejs yarnpkg webpack ffmpeg mkvtoolnix libvips-dev libxml2-dev postgresql-server-dev-all wget
+        apt-get -y install --no-install-recommends build-essential ruby ruby-dev ruby-bundler git nodejs yarnpkg webpack ffmpeg mkvtoolnix libvips-dev libxml2-dev libxslt-dev zlib1g-dev postgresql-server-dev-all wget
         ln -sf /usr/bin/yarnpkg /usr/bin/yarn
 
     - name: Install Ruby dependencies