Convert scache to abspath

I tried the run_HEPscore_default.sh benchmark with singularity version 3.8.1-1.el7 and Python 3.6.8. Singularity converts the docker image in the cache directory. The singularity image is at the right location. However, singularity does not find it again:

2021-09-01 16:10:11, hepscore.hepscore:_run_benchmark [DEBUG] Running  ['singularity', 'run', '-C', '-B', './suite_results/run_2021-09-01_1410/HEPSCORE/atla
s-gen-bmk/run0:/results', '-B', '/tmp:/tmp', '-u', 'docker://gitlab-registry.cern.ch/hep-benchmarks/hep-workloads/atlas-gen-bmk:v2.1', '-W', '--threads', '1
', '--events', '200']
2021-09-01 16:11:39, hepscore.hepscore:_check_return_code [ERROR] singularity returned code 255
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] singularity output logs:
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'2021/09/01 16:11:19  info unpack layer: sha256:270e4ea43d8117f050dda8be490522a0a7aee08baa83c
df52ed9c1e990defdef\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'2021/09/01 16:11:19  info unpack layer: sha256:f33a1c66a4ebfd0b798c29105e7576970b8c84f51d3ec
59496065e721c4b6771\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'2021/09/01 16:11:19  info unpack layer: sha256:73ec253603fb49ad6fe689ae9dd12d74b9163c6181d28
aa90a7f0d06216813ea\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'2021/09/01 16:11:19  info unpack layer: sha256:a99c9acff8f3522c9fc957ec183635b8062b536f69973
b10a0b74bf0973062ad\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'2021/09/01 16:11:19  info unpack layer: sha256:a048c662e39240ffc609cddfb8f96789d7d2c1bde39f9
d94f2443154b1fa616f\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'INFO:    Adding owner write permission to build path: /tmp/build-temp-000315788/rootfs\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'INFO:    Creating SIF file...\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'INFO:    Converting SIF file to temporary sandbox...\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b'FATAL:   while extracting suite_results/run_2021-09-01_1410/HEPSCORE/scache/cache/oci-tmp/56
ab03c9d543c1e8f9c2b2af6b59c0c7ff812ae07c6da94857dec592d93b9568: root filesystem extraction failed: extract command failed: FATAL:   could not open image /su
ite_results/run_2021-09-01_1410/HEPSCORE/scache/rootfs-102923069/tmp-rootfs-943671146: failed to retrieve path for /suite_results/run_2021-09-01_1410/HEPSCO
RE/scache/rootfs-102923069/tmp-rootfs-943671146: lstat /suite_results: no such file or directory\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] b': exit status 255\n'
2021-09-01 16:11:39, hepscore.hepscore:_run_benchmark [ERROR] running atlas-gen-bmk failed.  Exit status 255

Since the location of the singularity path is determined by an environment variable, a relative path can causes problems. Therefore, singularity recognizes the path always as an absolut path. However, with a conversion to an absolute path inside the benchmark code, it is working.