Pass experiments and set PACKAGE_MANAGER env variable on proxy

[AbhishekBhaskar]

This PR forwards experiments and PACKAGE_MANAGER to the proxy.

What

Enables the CLI to drive the proxy's upcoming egress allowlist feature by:

• Forwarding the job's experiments into the proxy config (so toggles like proxy_egress_observe / proxy_egress_enforce can be flipped locally).
• Setting the PACKAGE_MANAGER env var on the proxy container (so it loads the correct per-ecosystem allowlist defaults).

Why

Part of the Egress Allowlist in Proxy epic. The CLI previously only passed all_credentials + ca to the proxy, so the egress toggles couldn't be exercised via the CLI, and PACKAGE_MANAGER was empty — which would degrade the allowlist to GitHub-infra-only under enforce.

Changes

• internal/infra/config.go: add Experiments model.Experiment (json:"experiments,omitempty") to the proxy Config.
• internal/infra/proxy.go: populate proxyConfig.Experiments from params.Job.Experiments; add PACKAGE_MANAGER=<job.PackageManager> in proxyEnv().
• internal/infra/proxy_test.go: update proxyEnv call sites for the new signature and add a PACKAGE_MANAGER test.

Testing

• go build ./... and go test ./internal/... pass.
• Verified locally with dependabot update -f job.yml that a job's experiments block reaches the proxy config and PACKAGE_MANAGER is set on the container.