Fix usestdlibvars and whitespace linters issues.

[blackpiglet]

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Please add a summary of your change

Enable more linters for Velero.
This PR enable linter usestdlibvars and whitespace. All issues are fixed for the two new linters too.

Does your change fix a particular issue?

Fixes #(issue)

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• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov-commenter]

Codecov Report

Merging #6162 (2e44227) into main (14f31ee) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6162      +/-   ##
==========================================
- Coverage   41.29%   41.27%   -0.02%     
==========================================
  Files         251      251              
  Lines       23402    23379      -23     
==========================================
- Hits         9664     9650      -14     
+ Misses      12980    12970      -10     
- Partials      758      759       +1     

Impacted Files	Coverage Δ
internal/delete/delete_item_action_handler.go	74.66% <ø> (+0.98%)	⬆️
internal/hook/item_hook_handler.go	87.38% <ø> (+0.26%)	⬆️
internal/resourcepolicies/volume_resources.go	88.79% <ø> (ø)
...nal/resourcepolicies/volume_resources_validator.go	100.00% <ø> (ø)
pkg/backup/backup.go	52.24% <ø> (+0.10%)	⬆️
pkg/backup/item_backupper.go	73.62% <ø> (ø)
pkg/backup/item_collector.go	56.74% <ø> (+0.15%)	⬆️
pkg/backup/remap_crd_version_action.go	72.63% <ø> (ø)
pkg/client/config.go	19.04% <ø> (ø)
pkg/cmd/cli/backup/describe.go	0.00% <ø> (ø)
... and 15 more

... and 1 file with indirect coverage changes

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