nix.conf: move write for 'fallback', always set it

magic-nix-cache/src/main.rs

@@ -222,6 +222,11 @@ async fn main_cli() -> Result<()> {
         .open(&nix_conf_path)
         .with_context(|| "Creating nix.conf")?;
 
+    // always enable fallback, first
+    nix_conf
+        .write_all(b"fallback = true\n")
+        .with_context(|| "Setting fallback in nix.conf")?;
+
     let store = Arc::new(NixStore::connect()?);
 
     let narinfo_negative_cache = Arc::new(RwLock::new(HashSet::new()));

magic-nix-cache/src/pbh.rs

@@ -186,13 +186,7 @@ pub async fn setup_legacy_post_build_hook(
 
     /* Update nix.conf. */
     nix_conf
-        .write_all(
-            format!(
-                "fallback = true\npost-build-hook = {}\n",
-                post_build_hook_script.display()
-            )
-            .as_bytes(),
-        )
+        .write_all(format!("post-build-hook = {}\n", post_build_hook_script.display()).as_bytes())
         .with_context(|| "Writing to nix.conf")?;
 
     Ok(())