squash: Fix log file paths for beakerlib tests

Should be relative, not absolute when stored into `results.yaml`

old: data/guest/default-0/test/beakerlib/bad-1/output.txt
new: /var/tmp/tmt/run-059/plan/beakerlib/execute/data/guest/default-0/test/beakerlib/bad-1/output.txt

Regression discovered by `/tests/execute/basic`.

tmt/frameworks/beakerlib.py

@@ -48,7 +48,7 @@ def extract_results(
         log: list[Path] = []
         for filename in [tmt.steps.execute.TEST_OUTPUT_FILENAME, 'journal.txt']:
             if (invocation.path / filename).is_file():
-                log.append(invocation.path / filename)
+                log.append(invocation.relative_path / filename)
 
         # Check beakerlib log for the result
         beakerlib_results_filepath = invocation.path / 'TestResults'