Try two at toolchain

.cirrus.yml

@@ -173,10 +173,6 @@ task:
       env:
         TARGET: x86_64-fuchsia
         CHECK_TESTS: true
-    - name: illumos x86_64
-      env:
-        TARGET: x86_64-unknown-illumos
-        TOOLCHAIN: stable
   container:
     image: rust:1.40
   setup_script:
@@ -189,6 +185,21 @@ 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
+task:
+  name: illumos
+  env:
+    TARGET: x86_64-unknown-illumos
+    TOOLCHAIN: stable
+  container:
+    image: rust:1.40
+  setup_script:
+    - rustup toolchain install $TOOLCHAIN --profile minimal --target $TARGET
+  script:
+    - cargo +$TOOLCHAIN check --target $TARGET
+    - cargo +$TOOLCHAIN check --target $TARGET --release
+  before_cache_script: rm -rf $CARGO_HOME/registry/index
+
 # Redoxer is too unreliable, so we'll do a cross-build only
 # See also:
 # https://github.com/nix-rust/nix/issues/1258