Detecting Wrong Linux Proxy Via Env. Variables.

[JBalanza]

Describe the bug
When using a Linux beacon in an Environment where proxy is used, beacon gets the proxy from the HTTP_PROXY env var even tho the HTTPS_PROXY var is set and the C2 callback is made to a HTTPS website.

From implant/sliver/proxy/doc.go it details how the proxy is detected. Several environment variables like HTTPS_PROXY, HTTP_PROXY, FTP_PROXY, or ALL_PROXY are detected. The beacon is getting the HTTP_PROXY as first match if present, and ignoring the rest. Resulting into a wrong proxy setting since the C2 backend is HTTPS.

Steps to Reproduce

1. Generate a regular linux beacon with a target url that uses HTTPS. Debug version is optional but recommended.
2. In the victim machine, set different proxies for HTTP and HTTPS using HTTP_PROXY and HTTPS_PROXY variables respectively.
3. Execute the beacon. Note how the HTTP proxy is picked insted of the HTTPS.

Desktop (please complete the following information):

• OS: RHEL 8
• Version Sliver: Latest (v1.5.42)