fix: stop building codex-exec and codex-linux-sandbox binaries (#2036)

Release builds are taking awhile and part of the reason that we are
building binaries that we are not really using. Adding Windows binaries
into releases (https://github.com/openai/codex/pull/2035) slows things
down, so we need to get some time back.

- `codex-exec` is basically a standalone `codex exec` that we were
offering because it's a bit smaller as it does not include all the bits
to power the TUI. We were using it in our experimental GitHub Action, so
this PR updates the Action to use `codex exec` instead.
- `codex-linux-sandbox` was a helper binary for the TypeScript version
of the CLI, but I am about to axe that, so we don't need this either.

If we decide to bring `codex-exec` back at some point, we should use a
separate instances so we can build it in parallel with `codex`. (I think
if we had beefier build machines, this wouldn't be so bad, but that's
not the case with the default runners from GitHub.)