[DRE-maint] Bug#1123650: schleuder installation woes, episode 2

[Andrew Gallagher]

Further debugging info:

* run `apt install --reinstall` against each of the broken build 
dependencies
* run `apt install ruby-dev`
* run `gem pristine --all`

This gets rid of the first few the errors, but bumps up against a 
different error when trying to rebuild the `ruby-ffi` gem:


ERROR:  While executing gem ... (Gem::Package::FormatError)
     No such file or directory @ rb_sysopen - 
/usr/lib/x86_64-linux-gnu/rubygems-integration/3.3.0/cache/ffi-1.17.0.gem
root at fum:/etc/schleuder# ls 
/usr/lib/x86_64-linux-gnu/rubygems-integration/3.3.0/cache/ffi-1.17.0.gem
ls: cannot access 
'/usr/lib/x86_64-linux-gnu/rubygems-integration/3.3.0/cache/ffi-1.17.0.gem': 
No such file or directory
root at fum:/etc/schleuder# ls 
/usr/lib/x86_64-linux-gnu/rubygems-integration/3.3.0/cache/f*
/usr/lib/x86_64-linux-gnu/rubygems-integration/3.3.0/cache/ffi-1.17.0-x86_64-linux-musl.gem
root at fum:/etc/schleuder#


Where did that `musl` come from?