Switch from w7900 to using any persistent cache runner for CPU.

[saienduri]

This commit switches from w7900 to using any persistent cache runner for CPU model testing.
I also removed the persistent cache label from the w7900 runners because we don't have many and want to alleviate the load.
These jobs should now be running on the 16 mi250/mi300 runners we have

ci-exactly: build_packages,regression_test

[ScottTodd]

mi250:

============================== slowest durations ===============================
597.91s call     shark-test-suite-models/sdxl/test_unet.py::test_run_unet_cpu
292.26s call     shark-test-suite-models/sdxl/test_vae.py::test_run_vae_cpu
38.20s call     shark-test-suite-models/sdxl/test_unet.py::test_compile_unet_cpu
17.20s call     shark-test-suite-models/sdxl/test_clip.py::test_compile_clip_cpu

https://github.com/iree-org/iree/actions/runs/10498668335/job/29084321533#step:7:126

😬

might be an improvement over the w7900 if we at least have enough CPU cores / runners to run jobs in parallel

[saienduri]

Yeah mi250 is a bit slower than w7900. Not sure how the mi300 fares, but it was initially why I picked w7900. Specifically the w7900 has an AMD Ryzen Threadripper PRO 7975WX 32-Cores while the mi 250 has an AMD EPYC 7713 64-Core Processor. The mi300 has an AMD EPYC 9454 48-Core Processor, so I don't think that will be any better. We should get another CI machine up with just the threadripper CPU in the lab :)

[ScottTodd]

LGTM if the job times are reasonable and the runners are picking up the jobs as expected.

[saienduri]

The relevant tests worked as intended with the persistent-cache label, so merging