Drop slice-insert trick in 3-open-pr.py

Replace cmd[2:2] = ['-R', repo] with a plain cmd += ['-R', repo].
gh accepts flags in any order; appending is more idiomatic.

Author: trask

.github/scripts/flaky-test-fix/3-open-pr.py

@@ -107,7 +107,7 @@ def main():
            "--base", base,
            "--head", head]
     if repo:
-        cmd[2:2] = ["-R", repo]
+        cmd += ["-R", repo]
     if draft:
         cmd.append("--draft")
     print(f"Opening PR: {' '.join(cmd)}")