Add ec2_functests in ci.yml

[willieyz]

• During PR Add Runtime dispatch based on custom CPU capabilities function #607 , we found that currently there are no ec2_functests in mldsa-native
  Currently, mldsa-native does not include the ec2_functests section in its CI workflow.
  In mlkem-native, ec2_functests are functional tests that run on EC2 instances instead of GitHub runners. Running tests on EC2 provides more flexibility and control over the actual CPU.

This is not urgent for PR #607 and can be added later in a separate update.

[mkannwischer]

pq-code-package/mlkem-native#1277 should make this a bit easier.