As a workaround for https://github.com/NixOS/nixpkgs/issues/282509,
packages built with `emscriptenStdenv` need to set the `EM_CACHE`
environment variable to a folder in the build directory, so that
emscripten uses a copy of the sysroot cache instead of attempting to
write to the sysroot in the Nix store. However, due to the bug
https://github.com/emscripten-core/emscripten/issues/24404, the copied
cache has the same permission bits as the (readonly) one in the Nix
store, resulting in "Permission Denied" errors.
This bug has since been fixed in upstream emscripten, so we can backport
the relevant patches.
7101805a34