feat(tvix/derivation): make input_sources a BTreeSet

These need to be sorted anyways, so let's use the correct data structure for it.

Change-Id: I009c9989d7647dc1df716170f3680c981db6e4b2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7846
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/derivation/src/write.rs

@@ -118,7 +118,7 @@ pub fn write_input_derivations(
 
 pub fn write_input_sources(
     writer: &mut impl Write,
-    input_sources: &[String],
+    input_sources: &BTreeSet<String>,
 ) -> Result<(), fmt::Error> {
     writer.write_char(COMMA)?;