Add newline to end of plan json output (#283)

* Add newline to end of `plan` json output

This causes some shells (like sh) to continue their prompt right after
the closing brace, which is kinda meh.

* Add newline to end of `receipt.json`

`cat /nix/receipt.json` has the same issue as the previous commit, in
that it will bleed into the prompt for some shells that don't insert an
implicit newline.

* Add newline to end of fixture json documents

src/cli/subcommand/plan.rs

@@ -44,7 +44,7 @@ impl CommandExecute for Plan {
         };
 
         let json = serde_json::to_string_pretty(&install_plan)?;
-        tokio::fs::write(output, json)
+        tokio::fs::write(output, format!("{json}\n"))
             .await
             .wrap_err("Writing plan")?;
 

src/plan.rs

@@ -331,7 +331,7 @@ async fn write_receipt(plan: InstallPlan) -> Result<(), NixInstallerError> {
     let install_receipt_path = PathBuf::from(RECEIPT_LOCATION);
     let self_json =
         serde_json::to_string_pretty(&plan).map_err(NixInstallerError::SerializingReceipt)?;
-    tokio::fs::write(&install_receipt_path, self_json)
+    tokio::fs::write(&install_receipt_path, format!("{self_json}\n"))
         .await
         .map_err(|e| NixInstallerError::RecordingReceipt(install_receipt_path, e))?;
     Result::<(), NixInstallerError>::Ok(())