Forward the CI environment variable when running in Docker

We want to be able to adjust our configuration based on whether we are
running in CI, propagate this so our tests can use it.

library/compiler-builtins/libm/ci/run-docker.sh

@@ -28,6 +28,7 @@ run() {
     docker run \
         --rm \
         --user "$(id -u):$(id -g)" \
+        -e CI \
         -e RUSTFLAGS \
         -e CARGO_HOME=/cargo \
         -e CARGO_TARGET_DIR=/target \