mirror-from-saltstack-salt: fix prune pattern (was matching nothing)

[dwoz]

What does this PR do?

One-character fix to the prune step added in #70027. The pattern 'refs/pull/*' used with git for-each-ref matches zero refs — the * wildcard is treated as a literal, not a glob, in for-each-ref pattern arguments. The prune was a no-op, refs/pull/* still enumerated on push, and the HTTP 408 problem #70027 was meant to solve was still happening.

Correct syntax is the prefix without wildcard: refs/pull/. Same form already used in the before and after count lines directly adjacent.

Verification

Verified locally against a fresh git clone --bare --mirror of saltstack/salt:

With buggy pattern (current merged version of #70027):

$ git for-each-ref 'refs/pull/*' | wc -l
0
$ git for-each-ref refs/pull/ | wc -l
48905

With fix:

$ git for-each-ref refs/pull/ --format='delete %(refname)' | git update-ref --stdin
refs/pull/ pruned: 48905 -> 0
$ time git push --mirror ...salt-nightlies.git
Everything up-to-date
real    0m0.686s

Impact if not merged

The salt-nightlies mirror cron will fail nightly with HTTP 408 (same as before #70027 attempted a fix). The mirror still achieves ref sync at the git level (verified — master SHAs matched on both sides after the 408), so it's a workflow-ergonomics and runtime issue, not a data-correctness one. But nightly runs will always appear red.

Merge requirements satisfied?

• Docs — N/A.
• Changelog — none added.
• Tests — no automated tests; workflow is an Actions definition.

Commits signed with GPG?

No.