test: turn off multiplexing for cargo on cucumber

[delta1]

Description

Turn off http multiplexing 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)