hare: set HARECACHE in setupHook

If HARECACHE is unset, it hare will default to attempting to cache files
relative to $HOME, which isn't writable during a build. Set HARECACHE to
a suitable location so packages that are built with hare don't have to
manually handle this.

pkgs/development/compilers/hare/setup-hook.sh

@@ -1,3 +1,5 @@
+export HARECACHE="$NIX_BUILD_TOP/.harecache"
+
 addHarepath () {
     for haredir in third-party stdlib; do
         if [[ -d "$1/src/hare/$haredir" ]]; then