gnu: guile-dbi: Fix RPATH and dependencies.

Prior to this change, libguile-dbi.so was unable to find
libguile-dbd-sqlite3.so.

* gnu/packages/guile.scm (guile-dbi-bootstrap): New variable.
(guile-dbd-sqlite3)[propagated-inputs]: Remove guile-dbi.
[native-inputs]: Add guile-dbi-bootstrap.
(guile-dbi)[inputs]: Add guile-dbd-sqlite3.
[arguments]: Set RPATH using #:make-flags.