Setup dagger engines with buildkit gc disabled

[matipan]

This is a follow up from an internal discussion where @samalba has been investigating the performance of our CI and found that disabling bulidkit's GC has the potential of improving stability of the most heavy jobs. We want to test this in production for a few days and have side by side data to compare the performance and stability of the jobs. I've been running a few isolated tests here: https://github.com/dagger/dagger/actions/runs/9084221778, but we need more data in order to drive a conclusion regarding the GC.

In this PR we are adding two jobs for the engine-and-cli workflow:

• test-nogc
• testdev-nogc

This jobs will run in parallel and in a synchronous manner to baseline jobs. To implement it there were two minor changes introduced to _dagger_call:

• size-dind variable: this variable allows us to customize the runs-on for testdev which we couldn't so far.
• concurrency: allows the workflow dispatching dagger_call to control the concurrency group. We need this, otherwise nogc and baseline will step over each other's toes and cancel each other out.

[gerhard]

Left a few tiny comments. Also, this will need rebasing after these changes went it:

• Start removing mage 🎉 #7349