Concurrency setting in GHA cancels runs across PRs

[AnomalRoil]

Currently our GHA are using pull_request_target instead of pull_request:

kyber/.github/workflows/tests.yml

Line 4 in d0f3223

pull_request_target:

This has the unfortunate consequence that 2 different PRs can cancel each other jobs, because of how github.ref works:

Trigger event	pull_request	pull_request_target	push
gihub.ref	refs/pull/553/merge	refs/heads/master	refs/heads/test/pull_request_target
gihub.head_ref	test/pull_request_target	test/pull_request_target	-

• master: default branch
• test/pull_request_target: topic branch name
• 553: pull request number

I think the solution is to switch our concurrency groups from gihub.ref to gihub.head_ref:

kyber/.github/workflows/tests.yml

Lines 9 to 10 in d0f3223

	concurrency:
	group: ci-${{ github.ref }}-test