fix: respect proxy env vars in runner client (#962)

Fixes #957.

## Why
The runner builds a custom `http.Transport` for its RPC client. From first principles, once we stop using the default transport we also stop inheriting its default proxy resolution behavior, so `HTTP_PROXY`/`HTTPS_PROXY` are ignored unless we wire that behavior back explicitly.

## What
- set `Proxy: http.ProxyFromEnvironment` on the custom transport
- add a regression test that verifies `getHTTPClient` honors proxy environment variables

Reviewed-on: https://gitea.com/gitea/runner/pulls/962
Reviewed-by: ChristopherHX <38043+christopherhx@noreply.gitea.com>

internal/pkg/client/http.go

@@ -17,6 +17,7 @@ import (
 
 func getHTTPClient(endpoint string, insecure bool) *http.Client {
 	transport := &http.Transport{
+		Proxy:               http.ProxyFromEnvironment,
 		MaxIdleConns:        10,
 		MaxIdleConnsPerHost: 10, // All requests go to one host; default is 2 which causes frequent reconnects.
 		IdleConnTimeout:     90 * time.Second,