fix(arm64): shrink cargo mem footprint

[curiositycasualty]

The arm64 build was relatively silently failing while either doing the cargo build or just fetching the cargo registry index. This resolves that issue by taking steps to minimize cargo's memory footprint when building on arm64.

It also splits the atc-router install across the openresty build. Building atc_router.so before openresty, but installing atc_router.so after openresty-- since the openresty files themselves are not strictly required to build atc-router, just for placing the .so in the correct directory (as done by make).

Building atc-router sooner in build helps us fail faster when something with the atc-router goes awry.

[curiositycasualty]

Closing own stale PRs.