[codex] Eliminate direct writes to GITHUB_ENV

[ww-oai]

This eliminates some direct writes to ${GITHUB_ENV} in our custom (in-repo) actions. The idea behind this is twofold:

1. Reduce the amount of global mutation we do, to make individual steps less stateful (or where stateful, clearly initialized);
2. Eliminate a (minor) source of code injection risk, i.e. where a manipulated input induces us into setting environment variables that can be pivoted into code execution (things like LD_PRELOAD).

In practice (2) is not a real security risk at the moment, since the inputs to ${GITHUB_ENV} are not sufficiently attacker-controllable. But we should still address it for defense-in-depth.