fix: native gems are built with ExtensionTask.no_native=true

Specifically, following the advice of @kou at

  rake-compiler/rake-compiler#171

we set `Rake::ExtensionTask.no_native=true` within the
rake-compiler-dock container ("guest") so that:

- the ExtensionTask `cross_compiling` block is called
  - so that we don't package the dependencies' tarballs in /ports
  - so that we don't have mini_portile2 as a dependency (#2078)
  - so that we don't have an extra nokogiri.so/nokogiri.bundle built
    and packaged (#2076)
- we no longer have to hotfix rake-compiler at build time

This also will enable us to more easily do things like removing the C
extension source code from the native gem package (#2077).

concourse/tasks/gem-test/gem-build.sh

@@ -25,13 +25,7 @@ bundle install --local || bundle install
 bundle exec rake set-version-to-timestamp
 
 if [ -n "${BUILD_NATIVE_GEM:-}" ] ; then
-  # TODO remove after https://github.com/rake-compiler/rake-compiler/pull/171 is shipped
-  find /usr/local/rvm/gems -name extensiontask.rb | while read f ; do
-    echo "rewriting $f"
-    sudo sed -i 's/callback.call(spec) if callback/@cross_compiling.call(spec) if @cross_compiling/' $f
-  done
-
-  bundle exec rake gem:x86_64-linux:guest
+  bundle exec rake gem:x86_64-linux:guest FORCE_CROSS_COMPILING=true
 else
   # TODO we're only compiling so that we retrieve libxml2/libxslt
   # tarballs, we can do better a couple of different ways

rakelib/cross-ruby.rake

@@ -240,12 +240,10 @@ namespace "gem" do
   CROSS_RUBIES.find_all { |cr| cr.windows? || cr.linux? }.map(&:platform).uniq.each do |plat|
     desc "build native gem for #{plat} platform (host)"
     task plat do
-      # TODO remove `find` after https://github.com/rake-compiler/rake-compiler/pull/171 is shipped
       RakeCompilerDock.sh <<~EOT, platform: plat
         gem install bundler --no-document &&
         bundle &&
-        find /usr/local/rvm/gems -name extensiontask.rb | while read f ; do sudo sed -i 's/callback.call(spec) if callback/@cross_compiling.call(spec) if @cross_compiling/' $f ; done &&
-        rake gem:#{plat}:guest MAKE='nice make -j`nproc`'
+        rake gem:#{plat}:guest MAKE='nice make -j`nproc`' FORCE_CROSS_COMPILING=true
       EOT
     end
 
@@ -267,7 +265,6 @@ namespace "gem" do
   CROSS_RUBIES.find_all { |cr| cr.darwin? }.map(&:platform).uniq.each do |plat|
     desc "build native gem for #{plat} platform"
     task plat do
-      sh "find ~/.gem -name extensiontask.rb | while read f ; do sed -i '' 's/callback.call(spec) if callback/@cross_compiling.call(spec) if @cross_compiling/' \$f ; done"
       Rake::Task["native:#{plat}"].invoke
       Rake::Task["pkg/#{HOE.spec.full_name}-#{Gem::Platform.new(plat).to_s}.gem"].invoke
     end
@@ -328,6 +325,7 @@ else
   Rake::ExtensionTask.new("nokogiri", HOE.spec) do |ext|
     ext.lib_dir = File.join(*['lib', 'nokogiri', ENV['FAT_DIR']].compact)
     ext.config_options << ENV['EXTOPTS']
+    ext.no_native = (ENV["FORCE_CROSS_COMPILING"] == "true")
     ext.cross_compile  = true
     ext.cross_platform = CROSS_RUBIES.map(&:platform).uniq
     ext.cross_config_options << "--enable-cross-build"