build: Use install prefix in systemd files paths
When installing dconf into a prefix (like in jhbuild), we need to pass
dconf_prefix
to meson's get_pkgconfig_variable
to make sure that
systemd unit files are installed to the installation prefix, instead of
systemd's prefix (which will likely be your system's /usr).
See: https://github.com/systemd/systemd/issues/18082 See: https://github.com/systemd/systemd/commit/1c2c7c6cb3d92315624f3711114e86b0acfbce63