test: point at root in the local-mode apt failure

A local run fails apt-get update exactly like a local build does when
the user is not root; say so in the error instead of leaving a bare
failure (the chroot modes bootstrap their own root environment).

src/test/mod.rs

@@ -176,7 +176,16 @@ pub async fn run(opts: TestOptions<'_>) -> Result<TestRunReport, Box<dyn std::er
         env.insert("LOGNAME".to_string(), "root".to_string());
     }
 
-    configure_testbed(&ctx, &env, &series, &arch, &opts, view).await?;
+    configure_testbed(
+        &ctx,
+        &env,
+        &series,
+        &arch,
+        &opts,
+        matches!(bed, Bed::Local(_)),
+        view,
+    )
+    .await?;
 
     // Stage the tree and the binaries into the testbed. The copy also
     // protects the host tree: tests may write into it (rw-build-tree).
@@ -431,6 +440,7 @@ async fn configure_testbed(
     series: &str,
     arch: &str,
     opts: &TestOptions<'_>,
+    local_context: bool,
     view: &dyn BuildView,
 ) -> Result<(), Box<dyn std::error::Error>> {
     let mut sources = crate::apt::sources::load(Some(ctx.clone()))?;
@@ -509,7 +519,16 @@ async fn configure_testbed(
         .status()
         .map_err(|e| format!("apt-get update failed inside the testbed: {e}"))?;
     if !status.success() {
-        return Err("apt-get update failed inside the testbed".into());
+        return Err(format!(
+            "apt-get update failed inside the testbed{}",
+            if local_context {
+                " (a local run needs root to install packages: \
+                 use sudo, or the default chroot mode)"
+            } else {
+                ""
+            }
+        )
+        .into());
     }
 
     if opts.apt_upgrade {