Enable day 21 CI

Author: fwcd

.github/workflows/run.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-latest', 'macos-latest']
-        day: ['01', '02', '03', '04', '05', '06', '07', '08', '09', '10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20']
+        day: ['01', '02', '03', '04', '05', '06', '07', '08', '09', '10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20', '21']
         exclude:
         # TODO: Investigate why building Zig on macOS fails
         # https://github.com/fwcd/advent-of-code-2024/actions/runs/12202208638/job/34042374507