ci: use correct rust mounts

The old ones were too generic and clobbered over the new cargo and
rustup installations. What's strange is the new location installs to
where the old one _meant_ to go, but didn't actually. Fun sequence
of consequences.

.circleci/continue_config.yml

@@ -508,8 +508,8 @@ commands:
           command: |
             touch /tmp/docker.out
             mkdir /tmp/bashenv
-            sudo mkdir /rust
-            sudo chmod 777 /rust
+            sudo mkdir -p /rust/cargo/{git,registry}
+            sudo chmod -R 777 /rust
             mkdir /tmp/artifacts
             sudo chmod 777 /tmp/artifacts
             image=<< parameters.docker_image >>
@@ -530,7 +530,8 @@ commands:
                   -e CIRCLE_PROJECT_REPONAME \
                   -v $(pwd):/home/circleci/datadog \
                   -v /tmp/bashenv:/home/circleci/bashenv \
-                  -v /rust:/rust \
+                  -v /rust/cargo/git:/rust/cargo/git \
+                  -v /rust/cargo/registry:/rust/cargo/registry \
                   -v /tmp/artifacts:/tmp/artifacts \
                   $(if [ -n "${CARGO_TARGET_DIR:-}" ]; then echo -v ${CARGO_TARGET_DIR}:${CARGO_TARGET_DIR} -e CARGO_TARGET_DIR=${CARGO_TARGET_DIR}; fi) \
                   $image \