feat(tvix/derivation): derive Default for Derivation

Some of the fields in a Derivation struct stay empty, and manually
creating BTreeMap or vec for it is annoying.

Derive Default instead, so we can use the defaults instead of writing
more by hand.

Change-Id: I5d41b4b55c8187cb101eb4266451a470008e0067
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7788
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/derivation/src/derivation.rs

@@ -7,7 +7,7 @@ use std::{collections::BTreeMap, fmt, fmt::Write};
 use tvix_store::nixbase32::NIXBASE32;
 use tvix_store::store_path::{ParseStorePathError, StorePath, STORE_DIR};
 
-#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
 pub struct Derivation {
     #[serde(rename = "args")]
     pub arguments: Vec<String>,