nix: don't require the ZIG_LOCAL_CACHE_DIR to be set

NixOS does not set this during Nix package builds. Probably Nix's
caching got in the way of detecting this on my system.

fetch-zig-cache.sh

@@ -34,12 +34,6 @@ then
   exit 1
 fi
 
-if [ -z ${ZIG_LOCAL_CACHE_DIR+x} ]
-then
-  echo "must set ZIG_LOCAL_CACHE_DIR!"
-  exit 1
-fi
-
 zig build --fetch
 zig fetch git+https://github.com/zigimg/zigimg#3a667bdb3d7f0955a5a51c8468eac83210c1439e
 zig fetch git+https://github.com/mitchellh/libxev#f6a672a78436d8efee1aa847a43a900ad773618b

nix/build-support/check-zig-cache-hash.sh

@@ -26,9 +26,7 @@ elif [ "$1" != "--update" ]; then
 fi
 
 ZIG_GLOBAL_CACHE_DIR="$(mktemp --directory --suffix nix-zig-cache)"
-ZIG_LOCAL_CACHE_DIR="${ZIG_GLOBAL_CACHE_DIR}" 
 export ZIG_GLOBAL_CACHE_DIR
-export ZIG_LOCAL_CACHE_DIR
 
 # This is not 100% necessary in CI but is helpful when running locally to keep
 # a local workstation clean.