Don't download TCB from PCS in mock SGX mode

[matevz]

If internet is not available or intel.com site down, you get

level=error ts=2024-09-18T14:47:51.400155509Z caller=http.go:65 module=common/sgx/pcs/http msg="PCS request error" err="Get \"https://api.trustedservices.intel.com/sgx/certification/v4/tcb?fmspc=00606a000000&update=standard\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)" method=GET url="https://api.trustedservices.intel.com/sgx/certification/v4/tcb?fmspc=00606a000000&update=standard"

When compiling runtimes with --features debug-mock-sgx flag, don't download the TCB updates from the intel's site.

[kostko]

Moving this over to Oasis Core as it needs to be implemented here.