CI: add Illumos scheduled workflow

this is very initial build only implementation.

.github/workflows/illumos.yml

@@ -0,0 +1,23 @@
+name: Illumos
+
+on:
+  schedule:
+    - cron: "0 0 25 * *"
+
+jobs:
+  gcc:
+    runs-on: ubuntu-latest
+    if: ${{ github.repository_owner == 'haproxy' }}
+    permissions:
+      contents: read
+    steps:
+      - name: "Checkout repository"
+        uses: actions/checkout@v4
+
+      - name: "Build on VM"
+        uses: vmactions/solaris-vm@v1
+        with:
+          prepare: |
+            pkg install gcc make
+          run: |
+            gmake CC=gcc TARGET=solaris USE_OPENSSL=1 USE_PROMEX=1