Workflow for debugging the manifest file

[wshaos at posteo.net]

I understand that flatpak caches each module in the manifest. So if a 
module builds correctly the first time and you don't change it, the 
previous build will get reused when you build the flatpak again. 
Recently I was trying to build a flatpak for a project that takes about 
an hour to compile on my system. I was having trouble getting the 
post-install steps correct after the main compile (just copying / 
editing some text files). Each time I changed a post-install item, 
flatpak would go through the full one hour compile again. Is there any 
way to get flatpak to reuse the output from a previous attempt to build 
a module even when the previous build of that module exited with 
non-zero status? I understand why you would want to have a complete, 
clean build of the module for the final flatpak, but for debugging it 
would be nice to have a way to reuse all the compiled binaries while 
iterating on the build-commands and post-install entries.