fix: use http proxy for s3 access

I don't know why the AWS sdk disabled it by default. It would be nice
to have test coverage of the s3 store or proxies, but neither currently
exist.

Fixes: #433
Change-Id: If1e76169a3d66dbec2e926af0d0d0eccf983b97b

doc/manual/rl-next/http-proxy-for-s3.md

@@ -0,0 +1,10 @@
+---
+synopsis: HTTP proxy environment variables are now respected for S3 binary cache stores
+issues: [fj#433]
+cls: [1788]
+category: Fixes
+credits: jade
+---
+
+Due to "legacy reasons" (according to the AWS C++ SDK docs), the AWS SDK ignores system proxy configuration by default.
+We turned it back on.

src/libstore/s3-binary-cache-store.cc

@@ -140,6 +140,8 @@ ref<Aws::Client::ClientConfiguration> S3Helper::makeConfig(
     res->connectTimeoutMs = 5 * 1000;
     res->retryStrategy = std::make_shared<RetryStrategy>();
     res->caFile = settings.caFile;
+    // Use the system proxy env-vars in curl for s3, which is off by default for some reason
+    res->allowSystemProxy = true;
     return res;
 }