Use source for grpc and google-protobuf on Ruby preview

While the presence of a ruby-platform gem for these should be picked up
by bundler, sometimes it is not. Also, this makes it a bit more
future-proof, so that bundler doesn't attempt to pick a version that has
no ruby platform gem, and then proceed to fail.

Gemfile

@@ -76,7 +76,11 @@ gem 'opentracing', '>= 0.4.1'
 #       Since most of our customers won't have BUNDLE_FORCE_RUBY_PLATFORM=true, it's not something we want to add
 #       to our CI, so we just shortcut and exclude specific versions that were affecting our CI.
 if RUBY_PLATFORM != 'java'
-  if RUBY_VERSION >= '2.5.0' # Bundler 1.x fails to recognize that version >= 3.19.2 is not compatible with older rubies
+  if RUBY_DESCRIPTION =~ /^ruby \d+\.\d+\.\d+preview/
+    # binary gems for these have a max ruby version requirement that excludes preview versions
+    gem 'google-protobuf', ['~> 3.0', '!= 3.7.0', '!= 3.7.1'], platforms: ['ruby']
+    gem 'grpc', platforms: ['ruby']
+  elsif RUBY_VERSION >= '2.5.0' # Bundler 1.x fails to recognize that version >= 3.19.2 is not compatible with older rubies
     gem 'google-protobuf', ['~> 3.0', '!= 3.7.0', '!= 3.7.1']
   else
     gem 'google-protobuf', ['~> 3.0', '!= 3.7.0', '!= 3.7.1', '< 3.19.2']