Paket crash when --no-install is specified on update with generate_include_scripts

Description

Original report: https://github.com/fsharp/FAKE/issues/1672
But this reproduces with paket standalone as well...

This issue only reproduces when a clear state is used (ie packages, paket-files and paket.lock as well as .paket/load deleted):

paket.dependencies

source https://api.nuget.org/v3/index.json
generate_load_scripts: true
nuget Fake.Core.Targets prerelease

group External
    source https://api.nuget.org/v3/index.json

    nuget Google.Protobuf.Tools prerelease

.paket/paket.exe update --no-install --group Main

Paket failed with
-> Folder C:\proj\testing\packages\runtime.ubuntu.16.04-x64.runtime.native.System.Security.Cryptography.OpenSsl doesn't exist. Did you restore groups Main?

Expected behavior

Paket executes a restore even when --no-install is specified.

Actual behavior

Paket doesn't execute a restore and obviously fails when trying to generate load scripts.

Known workarounds

do a restore after the error.

Reproduce the issue from a clean state using paket.dependencies and .paket/paket.exe update --no-install --group Main, with the listed Main and External groups. Trace the update path that generates load scripts and verify that the command restores the required packages before generation, while preserving the intended --no-install behavior.