libstore: Don't default build-dir to temp-dir, store setting

If a build directory is accessible to other users it is possible to
smuggle data in and out of build directories. Usually this is only
a build purity problem, but in combination with other issues it can
be used to break out of a build sandbox. to prevent this we default
to using a subdirectory of nixStateDir (which is more restrictive).

(cherry picked from pennae Lix commit 55b416f6897fb0d8a9315a530a9b7f0914458ded)
(store setting done by roberth)

tests/functional/build-remote-trustless-should-fail-0.sh

@@ -12,7 +12,6 @@ requiresUnprivilegedUserNamespaces
 [[ $busybox =~ busybox ]] || skipTest "no busybox"
 
 unset NIX_STORE_DIR
-unset NIX_STATE_DIR
 
 # We first build a dependency of the derivation we eventually want to
 # build.

tests/functional/build-remote-trustless.sh

@@ -9,7 +9,6 @@ requiresUnprivilegedUserNamespaces
 [[ "$busybox" =~ busybox ]] || skipTest "no busybox"
 
 unset NIX_STORE_DIR
-unset NIX_STATE_DIR
 
 remoteDir=$TEST_ROOT/remote
 

tests/functional/build-remote.sh

@@ -8,7 +8,6 @@ requiresUnprivilegedUserNamespaces
 
 # Avoid store dir being inside sandbox build-dir
 unset NIX_STORE_DIR
-unset NIX_STATE_DIR
 
 function join_by { local d=$1; shift; echo -n "$1"; shift; printf "%s" "${@/#/$d}"; }
 

tests/functional/supplementary-groups.sh

@@ -14,7 +14,6 @@ execUnshare <<EOF
 
   # Avoid store dir being inside sandbox build-dir
   unset NIX_STORE_DIR
-  unset NIX_STATE_DIR
 
   setLocalStore () {
     export NIX_REMOTE=\$TEST_ROOT/\$1