Gemfile: pin nokogiri to main on aarch64-mingw-ucrt

sparklemotion/nokogiri#3530 is merged fixing the loader config on
windows-11-arm, but has not yet landed in a release.

Pin to `main` on the affected platform only, to unblock CI until a
fixed nokogiri release ships.

Author: flavorjones

Gemfile

@@ -19,3 +19,9 @@ platforms :mri, :windows do
 end
 
 gem "onigmo", platforms: :ruby
+
+# Until a nokogiri release includes sparklemotion/nokogiri#3530, aarch64-mingw-ucrt
+# source-builds of libxml2 fail in libtool. Pin to main on that platform only.
+if RUBY_PLATFORM =~ /aarch64.*mingw/
+  gem "nokogiri", github: "sparklemotion/nokogiri", branch: "main"
+end