azure: Uninstall previous rustc from builders if any

Looks like Azure has updated images recently to install Rust by default,
but that can interfere with our own compiler (for example Cargo's test
suite we think) so be sure to uninstall it before proceeding.

.azure-pipelines/steps/run.yml

@@ -102,6 +102,10 @@ steps:
 
 - bash: |
     set -e
+    # Remove any preexisting rustup installation since it can interfere
+    # with the cargotest step and its auto-detection of things like Clippy in
+    # the environment
+    rustup self uninstall -y || true
     if [ "$IMAGE" = "" ]; then
       src/ci/run.sh
     else