ref-manual/variables.rst: add a link for BB_USE_HOME_NPMRC

Documented in the BitBake manual.

(From yocto-docs rev: f58b8c80b7665ab5e67398a4ce4bfaa035114278)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -711,6 +711,9 @@ system and gives an overview of their function and contents.
    :term:`BB_TASKHASH`
       See :term:`bitbake:BB_TASKHASH` in the BitBake manual.
 
+   :term:`BB_USE_HOME_NPMRC`
+      See :term:`bitbake:BB_USE_HOME_NPMRC` in the BitBake manual.
+
    :term:`BB_VERBOSE_LOGS`
       See :term:`bitbake:BB_VERBOSE_LOGS` in the BitBake manual.