Switch to consuming JSON from GitHub for "latest commit touching file"

For some reason, https://github.com/xxx/yyy/commits/HEAD/zzz.atom has recently changed from returning XML by default to returning JSON, which from https://docs.github.com/en/rest/activity/feeds?apiVersion=2022-11-28 seems like it's intentional, so this adds an explicit `Accept:` header (the only explicit JSON one that seems to work) and uses `jq` to determine the latest commit instead of hacking up the XML via `grep`+`awk`.

See also docker-library/docker#472

(Additionally, this switches the file to use `wget` consistently instead of a mix of `wget` and `curl`)

tianon/update-cirros-pipeline.groovy

@@ -20,7 +20,7 @@ node {
 
 				version="$(< arches/version)"
 
-				updated="$(curl -fsSL "https://github.com/cirros-dev/cirros/commits/$version.atom" |tac|tac| sed -nre '/.*<updated>(.+)<[/]updated>.*/ { s//\\1/p; q }' | xargs -i'{}' date --date '{}' --rfc-2822 --utc)"
+				updated="$(curl -fsSL --header 'Accept: application/json' "https://github.com/cirros-dev/cirros/commits/$version.atom" | jq -r '.payload | first(.commitGroups[].commits[] | [ .committedDate, .authoredDate ] | sort | reverse[])' | xargs -i'{}' date --date '{}' --rfc-2822 --utc)"
 				export GIT_AUTHOR_DATE="$updated"
 				export GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"