Do not clean gem artifacts after build. Doing so can lead to rebuilds
and failures.

Index: lib/rb_sys/mkmf.rb
--- lib/rb_sys/mkmf.rb.orig
+++ lib/rb_sys/mkmf.rb
@@ -139,8 +139,6 @@ module RbSys
         \t$(INSTALL_PROG) $(DLLIB) $(RUBYARCHDIR)
 
         gemclean:
-        \t$(ECHO) Cleaning gem artifacts
-        \t-$(Q)$(RM_RF) $(RUBYGEMS_CLEAN_DIRS) 2> /dev/null || true
 
         install: #{builder.clean_after_install ? "install-so gemclean" : "install-so"}
 
