fix: enable socks proxy

[badeggg]

Summary
This pull request makes yarn can be used behind a socks proxy. Hence fix #6525.

Notice

• Since request library does not really handle socks proxy. I changed code to respect PROXY, HTTP_PROXY and HTTPS_PROXY environs in yarn. The reason for checking PROXY env is to keep consistent with npm
• Logic to handle noproxy config between npm and yarn is different(not changing in this pull request)
• I don't understand the text format of CHANGELOG.md, please help me to add a entry in the file

Test plan

# test file has following content

# step0 use changed yarn
yarn='/Users/zhaoxuxu/my_repos/yarn/bin/yarn'

# step1 make sure no proxy is configured
npm config delete proxy
npm config delete https-proxy
$yarn config delete proxy
$yarn config delete https-proxy
unset proxy PROXY  http_proxy HTTP_PROXY  https_proxy HTTPS_PROXY

# step2 set a env proxy
export proxy=socks5://127.0.01:1080 # or yarn config set proxy socks5://127.0.0.1:1080

# step3 make a request
mkdir testing
$yarn add lodash  # check your socks proxy server to validate that yarn is requesting via socks proxy

## repeat above steps except step2 should set proxy use different method

# output

zhaoxuxu:Desktop$  bash test 
yarn config v1.22.10
warning package.json: No license field
success Deleted "proxy".
✨  Done in 0.05s.
yarn config v1.22.10
warning package.json: No license field
success Deleted "https-proxy".
✨  Done in 0.06s.
yarn add v1.22.10
warning package.json: No license field
warning No license field
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 🔨  Building fresh packages...
warning No license field
success Saved 1 new dependency.
info Direct dependencies
└─ lodash@4.17.20
info All dependencies
└─ lodash@4.17.20
✨  Done in 8.18s.

[badeggg]

And please help me to figure out the auto test failing if you think this pull request is rational.

[CaledoniaProject]

Why is it still not merged?