build: Fix loading configuration file with default build args

Fix incorrect configuration path when the sysconfdir is relative to the
prefix argument:
fprintd-WARNING **: 12:22:38.816: Could not open "${prefix}/etc/fprintd.conf": No such file or directory

The path needs to be expanded before it's substituted.
2 jobs for master in 1 minute and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #486199
build_dev

00:01:19

passed #486198
build_stable

00:01:13