Update Illumos build workflows to use OmniOS VM

- Replaced OpenIndiana VM with OmniOS VM in Illumos build steps across `rust.yml` and `release.yml` workflows.
- Updated related workflow labels and comments for consistency.

.github/workflows/release.yml

@@ -7,12 +7,12 @@ on:
 
 jobs:
   build-illumos:
-    name: Build on Illumos (OpenIndiana)
+    name: Build on Illumos (OmniOS)
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
-    - name: Build in OpenIndiana VM
-      uses: vmactions/openindiana-vm@v1
+    - name: Build in OmniOS VM
+      uses: vmactions/omnios-vm@v1
       with:
         prepare: |
           pkg install -v developer/lang/rustc build-essential jq

.github/workflows/rust.yml

@@ -85,15 +85,15 @@ jobs:
           path: target/release/
           retention-days: 1
 
-  # Build on Illumos (OpenIndiana)
+  # Build on Illumos (OmniOS)
   build-illumos:
     name: Build (Illumos)
     needs: [format, clippy]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - name: Build in OpenIndiana VM
-        uses: vmactions/openindiana-vm@v1
+      - name: Build in OmniOS VM
+        uses: vmactions/omnios-vm@v1
         with:
           prepare: |
             pkg install -v developer/lang/rustc build-essential jq