Run s2n tests as soon as its saw binary is ready

[lisanna-dettwyler]

Parameterizing aspects like platform in GitHub Actions with a job matrix has the unfortunate downside of making dependent jobs wait longer than they really need to, since important artifacts are usually built before all of the job matrix variants finish.

Narrowing the dependency relationship in the build job to the linux artifact used for testing will enable a shorter turnaround time for the s2n tests, at the expense of some possible code duplication.