Fix do -p not waiting for external commands (#13881)

# Description
Similar to #13870 (thanks @WindSoilder), this PR adds a boolean which
determines whether to ignore any errors from an external command. This
is in order to fix #13876. I.e., `do -p` does not wait for externals to
complete before continuing.

# User-Facing Changes
Bug fix.

# Tests + Formatting
Added a test.

crates/nu-cmd-lang/src/core_commands/do_.rs

@@ -169,7 +169,7 @@ impl Command for Do {
                     && !matches!(caller_stack.stdout(), OutDest::Pipe | OutDest::Capture) =>
             {
                 if let ByteStreamSource::Child(child) = stream.source_mut() {
-                    child.set_exit_code(0)
+                    child.ignore_error();
                 }
                 Ok(PipelineData::ByteStream(stream, metadata))
             }