GHA: Update a tag of runner for aa_occlum_sgx workflow to sgx

Previously, a tag of the runner for the `aa_occlum_sgx` workflow
is `self-hosted` which is given by default for self-hosted runners
unless they use a `--no-default-labels` option for registration.
The tag would make the runner conflict with future self-hosted runners.

This commit updates the tag to `sgx`.

Signed-off-by: Hyounggyu Choi <Hyounggyu.Choi@ibm.com>

.github/workflows/aa_occlum_sgx.yml

@@ -17,7 +17,7 @@ on:
 
 jobs:
   occlum_sgx_ci:
-    runs-on: self-hosted
+    runs-on: sgx
     container:
       image: occlum/occlum:latest-ubuntu20.04
       options: --device /dev/sgx_enclave --device /dev/sgx_provision