Avoid double file buffering (#13780)

Co-authored-by: Johannes Müller <straightshoota@gmail.com>

src/crystal/system/unix/getrandom.cr

@@ -33,7 +33,7 @@ module Crystal::System::Random
       return unless urandom.info.type.character_device?
 
       urandom.close_on_exec = true
-      urandom.sync = true # don't buffer bytes
+      urandom.read_buffering = false # don't buffer bytes
       @@urandom = urandom
     end
   end

src/digest/digest.cr

@@ -213,6 +213,8 @@ abstract class Digest
   # Reads the file's content and updates the digest with it.
   def file(file_name : Path | String) : self
     File.open(file_name) do |io|
+      # `#update` works with big buffers so there's no need for additional read buffering in the file
+      io.read_buffering = false
       self << io
     end
   end

src/file.cr

@@ -797,8 +797,10 @@ class File < IO::FileDescriptor
     open(filename, mode, perm, encoding: encoding, invalid: invalid) do |file|
       case content
       when Bytes
+        file.sync = true
         file.write(content)
       when IO
+        file.sync = true
         IO.copy(content, file)
       else
         file.print(content)