fsync after writing Nix config to attempt to fix the flaky preserves_comment test (#448)

src/action/base/create_or_merge_nix_config.rs

@@ -434,6 +434,10 @@ impl Action for CreateOrMergeNixConfig {
                     e,
                 ))
             })?;
+        temp_file
+            .sync_all()
+            .await
+            .map_err(|e| Self::error(ActionErrorKind::Sync(temp_file_path.clone(), e)))?;
         tokio::fs::rename(&temp_file_path, &path)
             .await
             .map_err(|e| {

src/action/mod.rs

@@ -443,6 +443,8 @@ pub enum ActionErrorKind {
     Open(std::path::PathBuf, #[source] std::io::Error),
     #[error("Write path `{0}`")]
     Write(std::path::PathBuf, #[source] std::io::Error),
+    #[error("Sync path `{0}`")]
+    Sync(std::path::PathBuf, #[source] std::io::Error),
     #[error("Seek path `{0}`")]
     Seek(std::path::PathBuf, #[source] std::io::Error),
     #[error("Flushing `{0}`")]

src/cli/arg/instrumentation.rs

@@ -27,7 +27,7 @@ impl std::fmt::Display for Logger {
     }
 }
 
-#[derive(clap::Args, Debug)]
+#[derive(clap::Args, Debug, Default)]
 pub struct Instrumentation {
     /// Enable debug logs, -vv for trace
     #[clap(short = 'v', env = "NIX_INSTALLER_VERBOSITY", long, action = clap::ArgAction::Count, global = true)]