Add windows-gnu CI

Author: mati865

azure-pipelines.yml

@@ -41,6 +41,10 @@ jobs:
       x86_64-msvc:
         TOOLCHAIN: stable-x86_64-pc-windows-msvc
         OTHER_TARGET: i686-pc-windows-msvc
+      x86_64-gnu:
+        # FIXME: switch to stable when 1.44 is released
+        TOOLCHAIN: nightly-x86_64-pc-windows-gnu
+        OTHER_TARGET: i686-pc-windows-gnu
 
 - job: rustfmt
   pool: