[Backport perf-v15] fix(stress_thread): stop assuming current path

[mergify]

cause code assumed HDR to be located on $HOME, in some situations like in the intergation tests of c-s, it wasn't finding it and failing like the following:

FAILED unit_tests/test_cassandra_stress_thread.py::test_01_cassandra_stress -
       invoke.exceptions.UnexpectedExit: Encountered a bad command exit code!

Command: 'cp cs-hdr-write-l1-c0-k1-4bf4051d-abc0-4c0d-840d-f0af5c6b6e8a.hdr
          /home/ubuntu/scylla-cluster-tests/unit_tests/
          cs-hdr-write-l1-c0-k1-4bf4051d-abc0-4c0d-840d-f0af5c6b6e8a.hdr'
Exit code: 1

Stdout:

Stderr:

cp: cannot stat 'cs-hdr-write-l1-c0-k1-4bf4051d-abc0-4c0d-840d-f0af5c6b6e8a.hdr': No such file or directory

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit da6e8bb)

Parent PR: #9555