Add ctr to shell completion

[vitorsavian]

Proposed Changes

• Export the containerd.sock to the $CONTAINERD_ADDRESS env variable to use
  ctr without the need to use --address /run/k3s/containerd/containerd.sock

Types of Changes

• New Feature

Verification

• run rke2 and use the command

rke2 completion bash -i --ctr

• while rke2 is running

ctr --namespace k8s.io container ls

Testing

Linked Issues

• Add ctr to rke2 shell completion #6590

User-Facing Change

Further Comments

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 25.11%. Comparing base (e5caf09) to head (3f839c7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/cli/cmds/completion.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6709      +/-   ##
==========================================
- Coverage   25.18%   25.11%   -0.08%     
==========================================
  Files          33       33              
  Lines        2831     2839       +8     
==========================================
  Hits          713      713              
- Misses       2071     2079       +8     
  Partials       47       47              

Flag	Coverage Δ
inttests	9.54% <0.00%> (-0.03%)	⬇️
unittests	17.75% <0.00%> (-0.06%)	⬇️

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