Fix build bug, manifested as failure to find libguile-srfi-srfi-1-v-4

Fix build bug, manifested as failure to find libguile-srfi-srfi-1-v-4

* module/ice-9/boot-9.scm (top-repl): This use of @ is not safe, because
  it will cause (system repl repl) to be loaded at expand time, which
  will eventually cause (srfi srfi-1) to be loaded, which won't work
  with a fresh build tree because the helper lib isn't compiled. Gross.
  (load): Add a note about why this use of @ is safe.