Split build-su2-tsan into multiple jobs

[jblueh]

Overview

The thread-sanitizer enabled containers introduced in #17 take too long to build, building gcc takes around three hours and the time limit for an individual job is six hours. We need to build gcc for amd64 and arm64 and we can't do both it in a single job.

Let's see if this works better.

• We do not build the build-su2-tsan container image as part of the build-su2 job.
• We use different jobs to create single-platform container images for amd64 and arm64.
• Afterwards, we merge the single-platform container images.

I ensured that the same date_tag is used throughout the workflow.

Related work

continues #17