Bump nightly release branch to 2.20 (#666)

Author: jdblischak

.github/workflows/nightly.yml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        libtiledb_version: [release-2.19, dev]
+        libtiledb_version: [release-2.20, dev]
     steps:
       - name: Clone TileDB
         uses: actions/checkout@v3