cxrvh

--run commands in a sandbox

1

Closes: #348

## Linux distribution and version Fedora 32 ## Flatpak-builder version 1.0.10 ## Flatpak version 1.8.2 ## Description of the problem I've created a simple example which demonstrates the issue ([org.flatpak.builder-test.zip](https://github.com/flatpak/flatpak-builder/files/5469290/org.flatpak.builder-test.zip))....

Crash when included manifest is missing

7

## Linux distribution and version Fedora 30 ## Flatpak-builder version 1.0.9 ## Flatpak version 1.4.3 ## Description of the problem Crash when trying to build a manifest whose included manifests...