The pkgconfig files defines the wrong prefix: It is the build directory, while
it should be the installation prefix. This is caused by Makefiles replacing
`PREFIX` by `$(shell pwd)`.
See <https://gitlab.com/sequoia-pgp/sequoia/-/issues/502> for details.
* gnu/packages/sequoia.scm (sequoia)[fix-pkgconfig-file-substitutes]: New
phase.