Fix proxy support for http/https requests

[l0rda]

Hello,

The proxy support was missed when creating a new http.Client with custom Transport{}.
Therefore the "-proxy" option had no effect on http/https connections. This PR reverts to the standard behavior of the go http client with os environ support.

[j-baines]

oof. Thanks! Giving this a test.

[j-baines]

Before:

albinolobster@mournland:~/cve-2023-22527/nashorn$ ./build/cve-2023-22527_linux-arm64 -v -rhost google.com -rport 443 -s -proxy http://127.0.0.1:8080
time=2024-04-09T21:17:40.296-04:00 level=STATUS msg="Starting target" index=0 host=google.com port=443 ssl=true "ssl auto"=false
time=2024-04-09T21:17:40.296-04:00 level=STATUS msg="Validating Confluence target" host=google.com port=443
time=2024-04-09T21:17:40.646-04:00 level=ERROR msg="The target isn't recognized as Confluence, quitting" host=google.com port=443 verified=false

Resulted in:

albinolobster@mournland:~/Downloads/PolarProxy$ ./PolarProxy --httpconnect 8080
<6>Starting PolarProxy 0.9.7.0
<6>[8080] HTTP CONNECT proxy Listening on: 0.0.0.0:8080

After:

albinolobster@mournland:~/cve-2023-22527/nashorn$ ./build/cve-2023-22527_linux-arm64 -v -rhost google.com -rport 443 -s -proxy http://127.0.0.1:8080
time=2024-04-09T21:16:15.992-04:00 level=STATUS msg="Starting target" index=0 host=google.com port=443 ssl=true "ssl auto"=false
time=2024-04-09T21:16:15.992-04:00 level=STATUS msg="Validating Confluence target" host=google.com port=443
time=2024-04-09T21:16:16.420-04:00 level=ERROR msg="The target isn't recognized as Confluence, quitting" host=google.com port=443 verified=false

Resulted in:

albinolobster@mournland:~/Downloads/PolarProxy$ ./PolarProxy --httpconnect 8080
<6>Starting PolarProxy 0.9.7.0
<6>[8080] HTTP CONNECT proxy Listening on: 0.0.0.0:8080
<6>[8080] HTTP CONNECT proxy CONNECT proxy request from 127.0.0.1:45320 to google.com:443
<6>[8080] 127.0.0.1:8080 -> google.com:443 Connection request for google.com from 127.0.0.1:45320
<6>[8080] 127.0.0.1:8080 -> google.com:443 Action: DECRYPT
<6>[8080] HTTP CONNECT proxy CONNECT proxy request from 127.0.0.1:45322 to www.google.com:443
<6>[8080] 127.0.0.1:8080 -> www.google.com:443 Connection request for www.google.com from 127.0.0.1:45322
<6>[8080] 127.0.0.1:8080 -> www.google.com:443 Action: DECRYPT
<6>[8080] 127.0.0.1:8080 -> www.google.com:443 Stream closed by client (239 bytes read)
<6>[8080] 127.0.0.1:8080 -> google.com:443 Stream closed by client (205 bytes read)