avatar: reduce number of shards

Having too many shards extends the total testing time. 4 shards is a
good compromise.

Also, run all the shards and does not skip when one shard is failing

.github/workflows/python-avatar.yml

@@ -14,14 +14,10 @@ jobs:
     name: Avatar [${{ matrix.shard }}]
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
         shard: [
-           1/24,  2/24,  3/24,  4/24,
-           5/24,  6/24,  7/24,  8/24,
-           9/24, 10/24, 11/24, 12/24,
-          13/24, 14/24, 15/24, 16/24,
-          17/24, 18/24, 19/24, 20/24,
-          21/24, 22/24, 23/24, 24/24,
+           1/4,  2/4,  3/4,  4/4,
         ]
     steps:
       - uses: actions/checkout@v3