chore: cancel concurrent builds with native feature

[jawnsy]

Use the native 'concurrency' configuration feature to cancel
concurrent builds, rather than the cancel-workflow-action.
This also allows us to reduce permissions for the workflow.

[codecov]

Codecov Report

Merging #116 (f77eeda) into main (7cf686c) will increase coverage by 3.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   68.92%   71.93%   +3.00%     
==========================================
  Files          86       91       +5     
  Lines        3447     3670     +223     
  Branches       59       59              
==========================================
+ Hits         2376     2640     +264     
+ Misses        883      813      -70     
- Partials      188      217      +29     

Flag	Coverage Δ
unittest-go-macos-latest	68.13% <ø> (+0.31%)	⬆️
unittest-go-ubuntu-latest	69.80% <ø> (?)
unittest-go-windows-latest	67.38% <ø> (?)
unittest-js	74.72% <ø> (ø)

Impacted Files	Coverage Δ
coderd/userpassword/userpassword.go	63.41% <0.00%> (ø)
provisioner/terraform/parse.go	75.00% <0.00%> (ø)
provisioner/terraform/provision.go	57.62% <0.00%> (ø)
database/postgres/postgres.go	70.00% <0.00%> (ø)
provisioner/terraform/serve.go	66.66% <0.00%> (ø)
peer/channel.go	87.19% <0.00%> (+2.43%)	⬆️
peer/conn.go	79.68% <0.00%> (+3.90%)	⬆️
coderd/coderdtest/coderdtest.go	100.00% <0.00%> (+22.64%)	⬆️
database/migrate.go	52.38% <0.00%> (+52.38%)	⬆️
database/pubsub.go	77.08% <0.00%> (+77.08%)	⬆️

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[kylecarbs]

Neato!

[jawnsy]

docs here: https://docs.github.com/en/actions/using-jobs/using-concurrency#example-only-cancel-in-progress-jobs-or-runs-for-the-current-workflow