thumbnail: bind mount /etc/ to the sandbox

This is especially important for libstdc++ on distributions that don't have it directly in a libdir and the runtime linker doesn't look where needed without /etc/ (e.g. if libstdc++ is in a GCC per-version subdirectory handled via /etc/

If /etc/ is not available, the runtime linker will look only at a set of predetermined paths - as seen with LD_DEBUG=libs added to the bwrap call with "--setenv LD_DEBUG libs":

find [0]; searching search cache=/etc/ search path=/lib64:/usr/lib64 (system search path) trying file=/lib64/ trying file=/usr/lib64/

followed by:

/usr/bin/totem-video-thumbnailer: error while loading shared libraries: cannot open shared object file: No such file or directory

If /etc/ is available, it will use that for the paths:

find [0]; searching search cache=/etc/ trying file=/usr/lib/gcc/x86_64-pc-linux-gnu/8.2.0/

By bind mounting just that file out of /etc, we get it to work on such a system.

Closes: #81 (closed)

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