6df4b6bf introduced a workaround to allow Ubuntu and Debian packages to
work with default flags, since CPack does not adjust the rpath of the
package executables according to its CPACK_PACKAGING_INSTALL_PREFIX. [1]
However, this workaround caused a regression on those distributions not
using /usr as their CPACK_PACKAGING_INSTALL_PREFIX, such as Guix.
Until CPack is fixed, removing this line forces build-deb.yml to set a
non-default CMAKE_INSTALL_PREFIX that matches
CPACK_PACKAGING_INSTALL_PREFIX.
[1]: https://cmake.org/cmake/help/latest/variable/CPACK_PACKAGING_INSTALL_PREFIX.html