Auto merge of #117328 - lqd:cranelift-rocket, r=Mark-Simulacrum

pass `CODEGEN_BACKENDS` to docker

The backends to build are now defined in the `CODEGEN_BACKENDS` env var. It's correctly set in CI, but wasn't passed to docker, hence cg_clif wasn't actually built in #81746.

r? `@Kobzol:` I locally tried `CODEGEN_BACKENDS="cranelift" DEPLOY=1 src/ci/docker/run.sh dist-x86_64-linux` and this change was enough for `ci/run.sh` to read the env var.

So I'll try as-is and we'll see.

Author: bors