fix: Update to MorphoCloud/MorphoCloudWorkflow@15f28c9

List of MorphoCloudWorkflow changes:

```
$ git shortlog 4b3ffc6..15f28c9 --no-merges
Jean-Christophe Fillion-Robin (1):
      fix(cloud-config): Update to MorphoCloud/exosphere@b4a94fd4d
```

See MorphoCloud/MorphoCloudWorkflow@4b3ffc6...15f28c9

cloud-config

@@ -36,7 +36,7 @@ runcmd:
     # --checkout "{instance-config-mgt-repo-checkout}"
     # -e "{ansible-extra-vars}"
     rm -rf /opt/instance-config-mgt
-    exosphere_sha="3097269c0961caf198ff54b021f363f121bc2d6a" # morpho-cloud-portal-2024.07.17-78a7e2d93
+    exosphere_sha="b4a94fd4dcd265113ad7208e3c3f8feb4f85069a" # morpho-cloud-portal-2024.07.17-78a7e2d93
     ansible-pull \
       --url "https://github.com/MorphoCloud/exosphere.git" \
       --checkout "$exosphere_sha" \