config: fix data race in template's GroupHosts

config: fix data race in template's GroupHosts

==================
WARNING: DATA RACE
Read at 0x00c00015a5a0 by goroutine 25:
  ruderich.org/simon/safcm/cmd/safcm/config.ResolveHostGroups.func1()
      cmd/safcm/config/groups.go:130 +0x1c7
  ruderich.org/simon/safcm/cmd/safcm/config.ResolveHostGroups()
      cmd/safcm/config/groups.go:154 +0x521
  [...]

Previous write at 0x00c00015a5a0 by goroutine 21:
  slices.insertionSortOrdered[go.shape.string]()
      /usr/lib/go-1.24/src/slices/zsortordered.go:15 +0x2f9
  slices.pdqsortOrdered[go.shape.string]()
      /usr/lib/go-1.24/src/slices/zsortordered.go:75 +0x6c4
  slices.Sort[go.shape.[]string,go.shape.string]()
      /usr/lib/go-1.24/src/slices/sort.go:18 +0xd6
  ruderich.org/simon/safcm/cmd/safcm/config.(*templateArgs).GroupHosts()
      cmd/safcm/config/templates.go:116 +0x97
  [...]