ci: enable contextcheck

[michaelbeaumont]

Summary

This at least checks we don't make the mistake of missing an existing
context.

[codecov-commenter]

Codecov Report

Merging #3412 (12b20b2) into master (ff7a55f) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #3412      +/-   ##
==========================================
- Coverage   52.15%   52.14%   -0.02%     
==========================================
  Files         938      938              
  Lines       54587    54587              
==========================================
- Hits        28470    28464       -6     
- Misses      23886    23892       +6     
  Partials     2231     2231              

Impacted Files	Coverage Δ
pkg/core/config/manager/manager.go	50.00% <0.00%> (ø)
pkg/core/secrets/manager/global_manager.go	39.39% <0.00%> (ø)
pkg/core/secrets/manager/manager.go	36.98% <0.00%> (ø)
pkg/plugins/bootstrap/k8s/plugin.go	1.28% <0.00%> (ø)
.../plugins/runtime/k8s/controllers/pod_controller.go	41.79% <0.00%> (ø)
...gins/runtime/k8s/controllers/outbound_converter.go	67.16% <66.66%> (ø)
...g/plugins/runtime/k8s/controllers/pod_converter.go	64.00% <75.00%> (ø)
...ugins/runtime/k8s/controllers/ingress_converter.go	70.66% <83.33%> (ø)
pkg/hds/authn/callbacks.go	69.62% <100.00%> (ø)
pkg/plugins/secrets/k8s/store.go	75.30% <100.00%> (ø)
... and 6 more

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[jpeach]

LGTM