Explicitly set build-version
Explicitly set build-version

This was removed in
https://github.com/desktop/desktop/pull/307/files#r76084401 because we
thought `build-version` was inferred by `electron-packager`. But it
turns out it only infers `app-version`:
https://github.com/electron-userland/electron-packager/pull/449

BugTesterTest/desktops,BugTesterTest/desktops,hjobrien/desktop,j-f1/forked-desktop,shiftkey/desktop,kactus-io/kactus,shiftkey/desktop,kactus-io/kactus,BugTesterTest/desktops,gengjiawen/desktop,j-f1/forked-desktop,j-f1/forked-desktop,gengjiawen/desktop,artivilla/desktop,artivilla/desktop,artivilla/desktop,gengjiawen/desktop,desktop/desktop,artivilla/desktop,say25/desktop,BugTesterTest/desktops,shiftkey/desktop,shiftkey/desktop,hjobrien/desktop,say25/desktop,kactus-io/kactus,desktop/desktop,gengjiawen/desktop,j-f1/forked-desktop,hjobrien/desktop,say25/desktop,kactus-io/kactus,desktop/desktop,say25/desktop,hjobrien/desktop,desktop/desktop