The rpath of `dot_builtins` and `gvpack` was being tweaked in order to teach
them how to locate the plugins. But a more pervasive adjustment is needed to
force binaries installed in a custom location to preference their own adjacent
libraries instead system-wide ones that may also be installed.

For example, diffing `ldd` output on `gvpack` across this change:

  …
  -libgvc.so.6 => /lib/x86_64-linux-gnu/libgvc.so.6 (0x000071f2db7ba000)
  +libgvc.so.6 => /tmp/tmp.kavVvrjrnf/bin/../lib/libgvc.so.6 (0x0000766e364ac000)
  …
  -libcgraph.so.6 => /lib/x86_64-linux-gnu/libcgraph.so.6 (0x000071f2db7a3000)
  +libcgraph.so.6 => /tmp/tmp.kavVvrjrnf/bin/../lib/libcgraph.so.6 (0x0000766e36489000)

Note how prior to this change `gvpack` was incorrectly linked against the system
libgvc and libcgraph instead of the libraries it was built with. This kind of
mismatch can cause crashes and misbehavior, and is all the reason why Graphviz
binaries sometimes report their version number as an older...