build: Simplify file substitution

Since meson 0.50, configuration_data() can take a dictionary.
6 jobs for wip/dueno/meson in 7 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #367900
fedora:asan

00:07:27

passed #367902
fedora:coverage

00:06:03

67.7%
passed #367901
fedora:ubsan

00:05:10

passed #367899
fedora:Werror

00:05:31

 
  Deploy
passed #367903
pages

00:00:27

passed pages:deploy

00:00:00