SSL_connect: SSL_ERROR_SYSCALL behind a proxy

Trying to run cargo behind a proxy. Note that rustup immediately prior to this is successful, and curl is able to reach github without issues.

[root@fab7-compute-1 ~]# curl -o /dev/null https://github.com/
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 52814    0 52814    0     0  93135      0 --:--:-- --:--:-- --:--:-- 93310
[root@fab7-compute-1 ~]# cargo install click
    Updating registry `https://github.com/rust-lang/crates.io-index`
warning: spurious network error (2 tries remaining): [35] SSL connect error (OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to proxy.esl.cisco.com:80 ); class=Net (12)
warning: spurious network error (1 tries remaining): [35] SSL connect error (OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to proxy.esl.cisco.com:80 ); class=Net (12)
error: failed to fetch `https://github.com/rust-lang/crates.io-index`           

Caused by:
  [35] SSL connect error (OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to proxy.esl.cisco.com:80 ); class=Net (12)
[root@fab7-compute-1 ~]# env | grep -i proxy
http_proxy=https://proxy.esl.cisco.com:80
https_proxy=https://proxy.esl.cisco.com:80

Reproduce cargo install click with the shown http_proxy and https_proxy settings, comparing the successful curl and rustup requests. Trace Cargo's proxy and HTTPS connection handling to identify why this proxy produces SSL_ERROR_SYSCALL; done means the reported proxy configuration can fetch the crates.io index successfully.