Use /bin/echo instead of shell builtin echo

builtin echo of Solaris 10's sh doesn't expand `~`.
20181203T061807Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66160 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/dir/home_spec.rb

@@ -16,7 +16,7 @@ describe "Dir.home" do
 
   platform_is_not :windows do
     it "returns the named user's home directory, from the user database, as a string if called with an argument" do
-      Dir.home(ENV['USER']).should == `echo ~#{ENV['USER']}`.chomp
+      Dir.home(ENV['USER']).should == `/bin/echo ~#{ENV['USER']}`.chomp
     end
   end