Fix missing gem specs test

Apparently missing gem specs are only populated if the platform for the
declared gem is one of the platforms Bundler is installing for. Since
`:rbx` wasn't one of the added platforms, the gem was completely being
ignored.

However, we can't just add `:rbx` as a platform to the lockfile since
(a) it is not a platform supported by RubyGems anymore, and (b) it isn't
a platform that Sorbet has any support for.

Of the platforms supported by RubyGems, the one that we can also resolve
Sorbet against seems only to be the `:java` platform. So, adding the gem
that we expect to be missing as a `:java` platform gem and then adding
`:java` as a platform to the lockfile resurfaces the missing gem spec
again.

Author: paracycle

spec/tapioca/cli/gem_spec.rb

@@ -767,11 +767,15 @@ class Secret; end
 
         it "must not generate RBIs for missing gem specs" do
           @project.write_gemfile!(<<~GEMFILE, append: true)
-            platform :rbx do
+            platform :java do
               gem "sidekiq", "=7.1.2"
             end
           GEMFILE
 
+          # We need to add the platform to the lock file so that bundler includes the gem
+          # but it won't be materializable on the current platform (since we are not on JRuby)
+          @project.exec("bundle lock --add-platform java")
+
           @project.bundle_install!
 
           result = @project.tapioca("gem --all")