background: Unescape the URI of the background file

Fixes #130
5 jobs for background-uri in 7 minutes and 3 seconds (queued for 17 seconds)
latest
Status Job ID Name Coverage
  Build
passed #88547
librem5
build:native

00:01:34

 
  Test
passed #88548
librem5
test:native

00:01:25

passed #88549
librem5
test:smoke:one-output

00:02:20

passed #88550
librem5
test:smoke:two-outputs

00:02:06

 
  Package
passed #88551
librem5
package:deb

00:01:43