ci/appveyor: Fix Electron installation
ci/appveyor: Fix Electron installation

  Electron installation can fail silently  triggering errors later when trying
  to make use of it.
  As suggested in a comment (see https://github.com/electron/electron/issues/20731#issuecomment-546616376),
  manually launching the installation seems to do the trick.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop