Fix assignee username to lowercase 'copilot'

Co-authored-by: dorkmo <1923070+dorkmo@users.noreply.github.com>

Author: Copilot

.github/workflows/arduino-ci.yml

@@ -80,7 +80,7 @@ jobs:
             ---
             *This issue was automatically created by the Arduino CI workflow.*`,
               labels: ['bug', 'arduino', 'ci'],
-              assignees: ['Copilot']
+              assignees: ['copilot']
             });
             console.log(`Created issue #${issue.data.number}`)
 
@@ -148,6 +148,6 @@ jobs:
             ---
             *This issue was automatically created by the Arduino CI workflow.*`,
               labels: ['bug', 'arduino', 'ci'],
-              assignees: ['Copilot']
+              assignees: ['copilot']
             });
             console.log(`Created issue #${issue.data.number}`)