This reverts commit 09dde57e93.
Apparently, the rust-cbindgen bump wasn't the cause for the firefox
build error reported in https://github.com/NixOS/nixpkgs/pull/83247
(we could reproduce the build error even after
09dde57e93 applied).
For some reason it must have succeeded on hydra, as it's in the cache,
tricking us in believing 76458f89f4 broke
it initially.
So the build seems flaky of some sort - we haven't yet determined
whether it's luck, compiling with the right CPUs or something else. :-/
There's still some investigation to be done
(https://github.com/NixOS/nixpkgs/issues/84283), but no need to keep an
ineffective revert around.
1eae7fb887