[issue-template] Retain checklist in chip-level test issue

The `markdown` field is lost when the issue is created. Add the
checklist as a `textarea` instead.

Signed-off-by: Michael Munday <mike.munday@lowrisc.org>
diff --git a/.github/ISSUE_TEMPLATE/chip-level-test.yml b/.github/ISSUE_TEMPLATE/chip-level-test.yml
index 1dcfc07..4a11262 100644
--- a/.github/ISSUE_TEMPLATE/chip-level-test.yml
+++ b/.github/ISSUE_TEMPLATE/chip-level-test.yml
@@ -40,11 +40,11 @@
     attributes:
       label: Contact person
       description: GitHub username for a hardware engineer who can answer questions.
-  - type: markdown
+  - type: textarea
     id: checklist
     attributes:
+      label: Checklist
       value: |
-        # Checklist
         Please fill out this checklist as items are completed. Link to PRs and issues as appropriate.
         - [ ] Check if existing test covers most or all of this testpoint (if so, either extend said test to cover all points, or skip the next 3 checkboxes)
         - [ ] Device-side (C) component developed