CI: add a supervised device for e2e coverage of supervised-only features

[danielpaulus]

The e2e device pool (ganjalf / office01) currently has no supervised device, so a whole class of supervised-only features can't be verified in CI and get merged on inference + defensive coding rather than a real device test:

• Silent Wi-Fi profile install (Allow for Silent Install of Wifi Profile #837)
• isSupervised() detection — and specifically the exact GetCloudConfiguration response shape: is IsSupervised at the top level or nested under a CloudConfiguration key? We currently cannot confirm this without a supervised device (the existing prepare.go/erase.go code fetches the config but never reads a field out of it, and there are no fixtures).
• Supervised profile / wallpaper installs (AddProfileSupervised), MDM passcode / Screen-Time clearing, etc.
• ios prepare / supervision flows.

Ask

1. Add one supervised device to the self-hosted e2e runner pool and expose it via a new var (e.g. GO_IOS_E2E_SUPERVISED_DEVICES + the ..._LINUX_... twin), mirroring the existing device vars.
2. Gate supervised-only e2e tests on that var, and add coverage for at least: isSupervised(), silent Wi-Fi profile install, and one supervised profile install.

Context: surfaced while reviewing #837 — we couldn't confirm the GetCloudConfiguration IsSupervised location, so that PR is being merged with a defensive both-locations read instead of a device-verified one.