tor-browser-builder might want to not build rustc
As part of #25977 (moved), teor and I were looking into how rustc is being called and built, and we noticed that the binary for 1.24.1 is downloaded and then used to compile 1.25.0. The network team's current policy is to continue using whatever rustc is the current stable until "some distro we care about" releases a new OS version which freezes the rustc they distribute. Additionally, as of eight months or so (sometime between 1.19 and 1.20) ago, the latest nightly is used to compile the next night's nightly, so it's no longer always the case that the last version will compile the current version (meaning you're probably going to end up rabbitholing each time a new stable is released to figure out which nightly you need to somehow get a copy of to build with). Additionally, on a more theoretical note, I'm not certain what we're getting in terms of Trusting Trust attacks by building the compiler only once, versus just downloading the one we need to use.