CI: add i686-unknown-linux-gnu to cross tests

Author: danieldk

.github/workflows/rust.yml

@@ -28,6 +28,7 @@ jobs:
     strategy:
       matrix:
         target:
+          - i686-unknown-linux-gnu
           - mips64-unknown-linux-gnuabi64
     steps:
       - uses: actions/checkout@v1