macOS: Ugly hack to make the tests succeed

Sandboxes cannot be nested, so if Nix's build runs inside a sandbox,
it cannot use a sandbox itself. I don't see a clean way to detect
whether we're in a sandbox, so use a test-specific hack.

https://github.com/NixOS/nix/issues/1413

tests/common.sh.in

@@ -15,6 +15,10 @@ export NIX_STATE_DIR=$TEST_ROOT/var/nix
 export NIX_CONF_DIR=$TEST_ROOT/etc
 export NIX_MANIFESTS_DIR=$TEST_ROOT/var/nix/manifests
 export _NIX_TEST_SHARED=$TEST_ROOT/shared
+if [[ -n $NIX_STORE ]]; then
+    export _NIX_TEST_NO_SANDBOX=1
+fi
+export _NIX_IN_TEST=$TEST_ROOT/shared
 export NIX_REMOTE=$NIX_REMOTE_
 unset NIX_PATH
 export TEST_HOME=$TEST_ROOT/test-home