[rubygems/rubygems] Fix another incorrect removal of "ruby" platform from lockfile

When dependencies in path sources have changed, we'll be re-resolving,
and we can't really know whether the resolution will be valid or invalid
for the Ruby platform, so skip the removal in that case.

https://github.com/rubygems/rubygems/commit/afc3b0956f

lib/bundler/definition.rb

@@ -941,6 +941,7 @@ module Bundler
                 Bundler.local_platform == Gem::Platform::RUBY ||
                 !platforms.include?(Gem::Platform::RUBY) ||
                 (@new_platform && platforms.last == Gem::Platform::RUBY) ||
+                @path_changes ||
                 @dependency_changes ||
                 !@originally_locked_specs.incomplete_ruby_specs?(dependencies)
 

spec/bundler/install/gemfile/path_spec.rb

@@ -782,6 +782,52 @@ RSpec.describe "bundle install with explicit source paths" do
 
       expect(the_bundle).to include_gems "rack 0.9.1"
     end
+
+    it "does not remove existing ruby platform" do
+      build_lib "foo", "1.0", :path => lib_path("foo") do |s|
+        s.add_dependency "rack", "0.9.1"
+      end
+
+      lockfile <<~L
+        PATH
+          remote: #{lib_path("foo")}
+          specs:
+            foo (1.0)
+
+        PLATFORMS
+          #{lockfile_platforms("ruby")}
+
+        DEPENDENCIES
+          foo!
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+
+      bundle "lock"
+
+      expect(lockfile).to eq <<~G
+        PATH
+          remote: #{lib_path("foo")}
+          specs:
+            foo (1.0)
+              rack (= 0.9.1)
+
+        GEM
+          remote: #{file_uri_for(gem_repo1)}/
+          specs:
+            rack (0.9.1)
+
+        PLATFORMS
+          #{lockfile_platforms("ruby")}
+
+        DEPENDENCIES
+          foo!
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      G
+    end
   end
 
   describe "switching sources" do