Use /bin/echo instead of shell builtin echo

builtin echo of Solaris 10's sh doesn't expand `~`.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable10x/ruby-trunk/log/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
 

spec/ruby/core/file/expand_path_spec.rb

@@ -89,7 +89,7 @@ describe "File.expand_path" do
   platform_is_not :windows do
     before do
       @var_home = ENV['HOME'].chomp('/')
-      @db_home = `echo ~#{ENV['USER']}`.chomp
+      @db_home = `/bin/echo ~#{ENV['USER']}`.chomp
     end
 
     # FIXME: these are insane!