it's in ld-wrapper: pkgs/build-support/bintools-wrapper/ld-wrapper.sh:

        for path in "$dir"/*; do
            file="${path##*/}"
            if [ "${libs[$file]:-}" ]; then
                rpaths["$dir"]=1
                extraAfter+=(-rpath "$dir")
                break
            fi
        done

I believe dynamic linker is set inf a slightly different way: via nix-support/ld-set-dynamic-linker file indirection. From pkgs/build-support/bintools-wrapper/add-flags.sh:

if [ -z "$NIX_DYNAMIC_LINKER_@suffixSalt@" ] && [ -e @out@/nix-support/ld-set-dynamic-linker ]; then
    NIX_DYNAMIC_LINKER_@suffixSalt@="$(< @out@/nix-support/dynamic-linker)"
fi

and later in ld-wrapper: pkgs/build-support/bintools-wrapper/ld-wrapper.sh:

extraBefore+=("-dynamic-linker" "$NIX_DYNAMIC_LINKER_@suffixSalt@")

so if I'm getting this, that means some substitution step isn't working correctly right?

/nix/store/aj621ihplrq2bjan90mj0j8jzd8yykai-pkg-config-wrapper-0.29.2/nix-support/setup-hook: line 61: export: `NIX_@wrapperName@_TARGET_HOST_@suffixSalt@=1': not a valid identifier

Yeah, normally substitution happens in pkgs/stdenv/generic/setup.sh: consumeEntire content < "$hook"; substituteAllStream content "file '$hook'" >> "${!outputDev}/nix-support/setup-hook". I think that means your environment does not define wrapperName variable when you build the derivation with a hook. Normally it is defined:

nix-repl> pkg-config.setupHooks
[ /home/slyfox/dev/git/nixpkgs/pkgs/build-support/setup-hooks/role.bash /home/slyfox/dev/git/nixpkgs/pkgs/build-support/pkg-config-wrapper/setup-hook.sh ]

nix-repl> pkg-config.wrapperName
"PKG_CONFIG_WRAPPER"

Sounds like your cc-wrapper is missing fPIC. nixpkgs is very conservative when it comes to -fPIC: it adds it to all build units (whether those are libraries or not): pkgs/build-support/cc-wrapper/add-hardening.sh:

for flag in "${!hardeningEnableMap[@]}"; do
...
    pic)
      if (( "${NIX_DEBUG:-0}" >= 1 )); then echo HARDENING: enabling pic >&2; fi
      hardeningCFlagsBefore+=('-fPIC')

Unless you removed pic from default hardening options or are using a derivation that explicitly disables pic.