test: turn off multiplexing for cargo on cucumber (#3533)

Description
---

Turn off [http multiplexing](https://doc.rust-lang.org/cargo/reference/config.html#httpmultiplexing) for cargo on the integration tests. 

Trying to resolve spurious network errors 

```
warning: spurious network error (1 tries remaining): [92] Stream error in the HTTP/2 framing layer (HTTP/2 stream 201 was not closed cleanly before end of the underlying stream)
error: failed to download from `https://crates.io/api/v1/crates/rsa/0.3.0/download`

Caused by:
  [92] Stream error in the HTTP/2 framing layer (HTTP/2 stream 203 was not closed cleanly before end of the underlying stream)
``` 

Reference: rust-lang/cargo#6513 (comment)

.circleci/config.yml

@@ -182,6 +182,8 @@ jobs:
     docker:
       - image: *rust_image
     resource_class: medium
+    environment:
+      CARGO_HTTP_MULTIPLEXING: false
     steps:
       - checkout
       - cucumber-js