Error log with http_proxy in tiflash

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   tiup playground

2. What did you expect to see?

run

3. What did you see instead?

Logging errors to ~/.tiup/data/RwPeCSW/tiflash-0/log/tiflash_error.log
E0417 18:59:02.251581340  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI
E0417 18:59:02.254655366  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI
E0417 18:59:02.254730960  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI
E0417 18:59:02.254770249  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI
E0417 18:59:02.254814578  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI
E0417 18:59:02.254856870  114486 http_proxy.cc:64]           'socks5' scheme not supported in proxy URI

4. What version of TiUP are you using (tiup --version)?

tiup version v0.0.5 tiup
Go Version: go1.13
Git Branch: master
GitHash: 9a454f3
Build Time: 2020-04-16 10:33:50 +0800

Reproduce the issue with tiup playground, then inspect the generated tiflash_error.log and the reported http_proxy.cc:64 location. Trace how the http_proxy environment setting reaches TiFlash and verify behavior with the reported socks5 scheme. Done means the playground runs without repeated unsupported-proxy errors.