Commit e20232eab6 moved ZIG_GLOBAL_CACHE_DIR setup from an
addEnvHooks call into zigConfigurePhase. This broke all zig packages
that referenced $ZIG_GLOBAL_CACHE_DIR in postPatch or postUnpack,
since that variable is no longer set until configurePhase runs.
Move the symlink creation to postConfigure for all affected packages.
Based on the Nixpkgs used and the version of nixfmt-rfc-style in that
version, it's likely that not the correct version is used.
Update scripts should instead run within a Nixpkgs development shell
(`nix-shell`/`nix develop`/`direnv`), where the correct version of
`nixfmt` (although `treefmt` should be preferred) is always available.