net-mgmt/thanos: Build with '-tags slicelabels' to fix gRPC panics

Thanos in at least build version 0.41.0-1 causes panics once gRPC
calls to the sidecar or store component are made. This appears to be
an issue known in upstream [0], but the fix requires the use of the
-tags slicelabels Go build flags.

[0] https://github.com/thanos-io/thanos/issues/8543

PR: 294337
Tested by: Magnus Kaiser <freebsd@4xoc.com>
Approved by: db@, yuri@ (Mentors, implicit)
MFH: 2026Q2