checkupdates-trigger: use a working parallels version

GNU parallels in Debian is 4 years out of date, missing about 40
releases and not updated in sid either.
The version in Debian claims to support the -X option for
splitting arguments in $jobs buckets but generates completely bogus jobs,
where the first job contains something like 80% of all arguments and the
other jobs contain much less.
It also generates 11 jobs when we asked for 10... :-/.

Also simplify the setup further by using parallel's {/.} replacement
pattern, which strips leading path's and extensions instead of bash
pattern substitution magic.

.gitlab-ci.yml

@@ -76,15 +76,11 @@ checkupdates_trigger:
       - schedules
     variables:
       - $CHECKUPDATES == "true"
-  image: debian:buster-slim
+  image: archlinux:latest
   before_script:
-    - apt-get update
-    - apt-get install -qy --no-install-recommends parallel curl ca-certificates
+    - pacman -Sy --noconfirm parallel
   script:
-    - files=( metadata/*.yml )
-    - files=( ${files[@]#metadata/} )
-    - files=( ${files[@]%.yml} )
-    - parallel --verbose --delay 90 -X --jobs 10 'curl -X POST -F "token=$CI_JOB_TOKEN" -F "ref=master" -F "variables[CHECKUPDATES]=true" -F "variables[CHECKUPDATES_APPIDS]= {} " https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/trigger/pipeline' ":::" ${files[@]}
+    - parallel --verbose --delay 90 -X --jobs 10 'curl -X POST -F "token=$CI_JOB_TOKEN" -F "ref=master" -F "variables[CHECKUPDATES]=true" -F "variables[CHECKUPDATES_APPIDS]= {/.} " https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/trigger/pipeline' ":::" metadata/*.yml
 
 checkupdates_runner:
   image: registry.gitlab.com/fdroid/ci-images-checkupdates:latest