Fix regex resolver corrupting the users scanner cache

[SungJin1212]

The regex resolver builds its user list with append(active, deleting...) followed by
an in-place sort.Strings. When -blocks-storage.users-scanner.cache-ttl is set, the
users scanner returns the very same slices on every scan, so sorting in place corrupts
the cached active list.

For example:

sync	knownUsers
1	[a-del b-1 b-2 b-3]
2	[a-del a-del b-1 b-2]
3	[a-del a-del a-del b-1]
4	[a-del a-del a-del a-del]
...

Tenants silently disappear from federated query results until the scanner cache expires.

This PR fixes it by copying the scanner result into a newly allocated slice before
sorting, so the sort only touches memory the resolver owns.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags