Modifying squashfs usage for `guix pack` for supporting singularity

[Josh Marshall]

I ran into some trouble today when trying to package an image for singularity 
on CentOS7.  nxck suggested I go into the cdoe and try adding the "-noappend" 
option to mksquashfs in 'guix/guix/scripts/pack.scm'.  While looking at it, it 
seems that it performs a multistep process to modify the squashfs image, which 
kind of goes against the design of the utility.  A SO page ( 
https://unix.stackexchange.com/a/459372 ) suggested the full hierarchy be 
created and then added in a single invocation.  Does anyone have any opinions 
on such a change or useful insights?
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