Rename and protect BufferedSink::write

The `write` name is ambiguous and could lead to some funny bugs like
https://github.com/NixOS/nix/pull/8173#issuecomment-1500009480. So
rename it to the more explicit `writeUnbuffered`.
Besides, this method shouldn't be (and isn't) used outside of the class
implementation, so mark it `protected`.

This makes it more symetrical to `BufferedSource` which uses a
`protected readUnbuffered` method.

src/libutil/compression.hh

@@ -12,7 +12,7 @@ namespace nix {
 struct CompressionSink : BufferedSink, FinishSink
 {
     using BufferedSink::operator ();
-    using BufferedSink::write;
+    using BufferedSink::writeUnbuffered;
     using FinishSink::finish;
 };