[deps] Use Own Prebuilt Spellchecker

[nsakaimbo]

In order to pre-empt the update of Calypso to node v12, we need to point electron-spellchecker at our own prebuilt fork to ensure prebuilds are fetched for the current version of Electron and not the environment's version of node (as set from Calypso's .nvmrc).

If we upgrade Calypso to latest, the app as-is will fail to build as by default prebuild-install will fetch binaries against the host node environment and not Electron. While the Electron upgrade is pending, I'm forcing prebuild-install to fetch upstream prebuilds for wp-desktop's current version of Electron here, here and here.

prebuild-install WARN install No prebuilt binaries found 
(target=12.13.1 runtime=node arch=x64 platform=darwin)
// ...
// ...
// ...
// BUILD FAILURE

Worth noting: This is yet another reason we shouldn't be using prebuilds in the first place.

[belcherj]

Ship it.