Attempt three

.cirrus.yml

@@ -185,16 +185,15 @@ task:
     # they don't build on all platforms.
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
-# illumos toolchain isn't available on 1.40
+# illumos toolchain isn't available via rustup until 1.50
 task:
   name: illumos
   env:
     TARGET: x86_64-unknown-illumos
-    TOOLCHAIN: stable
   container:
-    image: rust:1.40
+    image: rust:1.50
   setup_script:
-    - rustup toolchain install $TOOLCHAIN --profile minimal --target $TARGET
+    - rustup target add $TARGET
   script:
     - cargo +$TOOLCHAIN check --target $TARGET
     - cargo +$TOOLCHAIN check --target $TARGET --release