Fix storing of stdout/stderr in bootstrap commands that failed to start

Before, their stdout/stderr was forcefully set to `None`, even if the corresponding command tried to capture output.

src/bootstrap/src/lib.rs

@@ -1005,7 +1005,7 @@ Executed at: {executed_at}"#,
             \nIt was not possible to execute the command: {e:?}"
                 )
                 .unwrap();
-                CommandOutput::did_not_start()
+                CommandOutput::did_not_start(stdout, stderr)
             }
         };
         if !output.is_success() {

src/bootstrap/src/utils/exec.rs

@@ -231,8 +231,18 @@ pub struct CommandOutput {
 
 impl CommandOutput {
     #[must_use]
-    pub fn did_not_start() -> Self {
-        Self { status: CommandStatus::DidNotStart, stdout: None, stderr: None }
+    pub fn did_not_start(stdout: OutputMode, stderr: OutputMode) -> Self {
+        Self {
+            status: CommandStatus::DidNotStart,
+            stdout: match stdout {
+                OutputMode::Print => None,
+                OutputMode::Capture => Some(vec![]),
+            },
+            stderr: match stderr {
+                OutputMode::Print => None,
+                OutputMode::Capture => Some(vec![]),
+            },
+        }
     }
 
     #[must_use]