Don't fallback to tempdir when required directories exist.

When home directory is not writable, but the required .gem and .bundle
are, we should use them instead of falling back to use tempdirs.

This creates a workaround for more restrictive setups using Omnibus
Docker or any hardened setup, to overcome the annoyances introduced by rubygems#4951.

lib/bundler.rb

@@ -159,12 +159,13 @@ def ruby_scope
     def user_home
       @user_home ||= begin
         home = Bundler.rubygems.user_home
+        user_dirs = %w[.bundle .gem].map {|path| Pathname.new(home).join(path)}
 
         warning = if home.nil?
           "Your home directory is not set."
         elsif !File.directory?(home)
           "`#{home}` is not a directory."
-        elsif !File.writable?(home)
+        elsif !File.writable?(home) && user_dirs.any? {|path| !path.directory? || !path.writable?}
           "`#{home}` is not writable."
         end