Add some jobs to .appveyor.yml

.appveyor.yml

@@ -2,16 +2,24 @@ environment:
   matrix:
   - TARGET: x86_64-pc-windows-msvc
     CHANNEL: nightly
+  - TARGET: x86_64-pc-windows-msvc
+    CHANNEL: beta
   - TARGET: x86_64-pc-windows-msvc
     CHANNEL: stable
   - TARGET: x86_64-pc-windows-msvc
     CHANNEL: 1.24.1
   - TARGET: x86_64-pc-windows-gnu
     CHANNEL: stable
+  - TARGET: x86_64-pc-windows-gnu
+    CHANNEL: beta
   - TARGET: i686-pc-windows-msvc
-    CHANNEL: nightly
+    CHANNEL: stable
+  - TARGET: i686-pc-windows-msvc
+    CHANNEL: beta
   - TARGET: i686-pc-windows-gnu
-    CHANNEL: nightly
+    CHANNEL: stable
+  - TARGET: i686-pc-windows-gnu
+    CHANNEL: beta
 
 install:
   - ps: >-