Move illumos to Ubuntu 22.04 (#1114)

It was accidentally dropped in fd83c7a43e.

src/ubuntu/22.04/cross/illumos/Dockerfile

@@ -0,0 +1,16 @@
+ARG ROOTFS_DIR=/crossrootfs/x64
+
+FROM mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-22.04-crossdeps-local AS builder
+ARG ROOTFS_DIR
+
+# Obtain arcade scripts used to build rootfs
+RUN git config --global user.email builder@dotnet-buildtools-prereqs-docker && \
+    git clone --depth 1 --single-branch https://github.com/dotnet/arcade /scripts
+
+# Build the rootfs
+RUN /scripts/eng/common/cross/build-rootfs.sh x64 illumos --skipunmount
+
+FROM mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-22.04-crossdeps-local
+ARG ROOTFS_DIR
+
+COPY --from=builder $ROOTFS_DIR $ROOTFS_DIR

src/ubuntu/manifest.json

@@ -330,6 +330,19 @@
             }
           ]
         },
+        {
+          "platforms": [
+            {
+              "dockerfile": "src/ubuntu/22.04/cross/illumos",
+              "os": "linux",
+              "osVersion": "jammy",
+              "tags": {
+                "ubuntu-22.04-cross-illumos-$(System:TimeStamp)-$(System:DockerfileGitCommitSha)": {},
+                "ubuntu-22.04-cross-illumos$(FloatingTagSuffix)": {}
+              }
+            }
+          ]
+        },
         {
           "platforms": [
             {