config: fall back to $USER if username couldn't be obtained by libc

If jj is compiled against musl libc, not all name services are available, and
getpwuid() can return null even if the system is configured properly. That's
the problem reported as jj-vcs#5231. Suppose operation.username exists mainly for
logging/tracing purposes, it should be better to include something less reliable
than leaving the field empty.

This patch also removes TODO comment about empty hostname/username. It's
unlikely that the hostname is invalid (as that would cause panic on older jj
versions), and $USER would probably be set on Unix.

cli/src/config.rs

@@ -444,7 +444,6 @@ const OP_USERNAME: &str = "operation.username";
 /// Environment variables that should be overridden by config values
 fn env_base_layer() -> ConfigLayer {
     let mut layer = ConfigLayer::empty(ConfigSource::EnvBase);
-    // TODO: warn if hostname/username is empty after loading config files?
     if let Ok(value) = whoami::fallible::hostname()
         .inspect_err(|err| tracing::warn!(?err, "failed to get hostname"))
     {
@@ -454,6 +453,10 @@ fn env_base_layer() -> ConfigLayer {
         .inspect_err(|err| tracing::warn!(?err, "failed to get username"))
     {
         layer.set_value(OP_USERNAME, value).unwrap();
+    } else if let Ok(value) = env::var("USER") {
+        // On Unix, $USER is set by login(1). Use it as a fallback because
+        // getpwuid() of musl libc appears not (fully?) supporting nsswitch.
+        layer.set_value(OP_USERNAME, value).unwrap();
     }
     if !env::var("NO_COLOR").unwrap_or_default().is_empty() {
         // "User-level configuration files and per-instance command-line arguments