gnu: node: Update to 6.8.0.

Remove <https://debbugs.gnu.org/23744> and
<https://debbugs.gnu.org/23723

> workaround.

* gnu/packages/node.scm (node): Update to 6.8.0.
  (node)[arguments]: Disabled more tests. Remove custom 'patch-shebangs'
  phase. Manually patch npm script shebang in new 'patch-npm-shebang'
  phase.
* gnu/packages/patches/node-9077.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add patches.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>