meson: correctly set -DSANDBOX_SHELL if we have it

The statically embedded busybox is not required for Lix to work, but
package.nix explicitly sets this, which was accidentally being ignored.

Change-Id: Ieeff830ac7d1f5fabe84d1a6cfd82f13d79035bf

src/libstore/meson.build

@@ -156,6 +156,12 @@ cpp_str_defines = {
   'NIX_MAN_DIR':   mandir,
 }
 
+if busybox.found()
+  cpp_str_defines += {
+    'SANDBOX_SHELL': busybox.full_path()
+  }
+endif
+
 cpp_args = []
 
 foreach name, value : cpp_str_defines