Run rustup to install toolchain

With rustup >1.28.0 instaling the toolchain
requires an explicit command

Author: Remi Delmas

.github/workflows/verify-std-check.yml

@@ -50,6 +50,7 @@ jobs:
       - name: Build Kani
         working-directory: kani
         run: |
+          rustup toolchain install
           cargo build-dev
           echo "$(pwd)/scripts" >> $GITHUB_PATH