workflows: Include illumos in stable builds

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

Author: kinnison

.github/workflows/linux-builds-on-stable.yaml

@@ -28,6 +28,7 @@ jobs:
           - arm-unknown-linux-gnueabihf   # skip-pr skip-master
           - x86_64-unknown-freebsd        # skip-pr skip-master
           - x86_64-unknown-netbsd         # skip-pr skip-master
+          - x86_64-unknown-illumos        # skip-pr skip-master
           - powerpc-unknown-linux-gnu     # skip-pr skip-master
           - powerpc64le-unknown-linux-gnu # skip-pr skip-master
           - mips-unknown-linux-gnu        # skip-pr skip-master