fix: allow overriding DEVELOPER_DIR

[codebytere]

Allow users to override DEVELOPER_DIR.

Previously, if DEVELOPER_DIR was overridden, e load-xcode would try to switch it back every time.

[MarshallOfSound]

Should we let DEVELOPER_DIR override our xcode? Or should we make people set something like ELECTRON_BUILD_TOOLS_DEVELOPER_DIR instead thus ensuring that CI / build systems that already set the standard dev-dir env var don't lose cache hits.