build dist for x86_64-unknown-illumos

This change creates a new Docker image, "dist-x86_64-illumos", and sets
things up to build the full set of "dist" packages for illumos hosts, so
that illumos users can use "rustup" to install packages.  It also
adjusts the manifest builder to expect complete toolchains for this
platform.

.github/workflows/ci.yml

@@ -309,6 +309,9 @@ jobs:
           - name: dist-x86_64-freebsd
             os: ubuntu-latest-xl
             env: {}
+          - name: dist-x86_64-illumos
+            os: ubuntu-latest-xl
+            env: {}
           - name: dist-x86_64-linux
             os: ubuntu-latest-xl
             env: {}