gha: fix /backport command

[andrewhsu]

jira: PESDLC-2118

The /backport command is broken if there is more than 1 commit in a PR because the list of git commits have newlines:

bash$ backport_commits=$(gh api "repos/redpanda-data/redpanda/pulls/23492/commits" --paginate --jq '.[].sha[0:10]')
bash$ echo "backport_commits=$backport_commits"
backport_commits=c254729c5a
9a5f7d7d58
2bc21e9516
c9223a24ae

This patch fixes by having the commits separated by spaces:

bash$ backport_commits=$(gh api "repos/redpanda-data/redpanda/pulls/23492/commits" --paginate --jq '[.[].sha[0:10]]|join(" ")')
bash$ echo "backport_commits=$backport_commits"
backport_commits=c254729c5a 9a5f7d7d58 2bc21e9516 c9223a24ae

see #23492 (comment)

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.2.x
• v24.1.x
• v23.3.x

Release Notes

• none

[oleiman]

lgtm.

am i correct to assume this also fixes the missing commit sha's in #23573?