Set entrypoint jar to absolute directory to allow customizing working dir

[arminzavada]

Previous specification does not allow customizing the working directory in the container without breaking the default entrypoint.

[github-actions]

Benchexec test report for a selection of SV-Benchmarks (correct / incorrect / all):

Rundefinition	COMPLEX	EMERGENT	HORN
SV-COMP25_no-data-race	✅ (59 / 0 / 89) HTML/CSV	✅ (31 / 0 / 66) HTML/CSV	✅ (20 / 0 / 83) HTML/CSV
SV-COMP25_no-overflow	✅ (16 / 0 / 47) HTML/CSV	❓ (0 / 0 / 37) HTML/CSV	❓ (0 / 0 / 88) HTML/CSV
SV-COMP25_termination	✅ (1 / 0 / 202) HTML/CSV	✅ (1 / 0 / 205) HTML/CSV	✅ (14 / 0 / 114) HTML/CSV
SV-COMP25_unreach-call	✅ (18 / 0 / 95) HTML/CSV	✅ (11 / 0 / 87) HTML/CSV	✅ (10 / 0 / 190) HTML/CSV
SV-COMP25_valid-memcleanup	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV
SV-COMP25_valid-memsafety	✅ (34 / 0 / 319) HTML/CSV	✅ (2 / 0 / 273) HTML/CSV	❓ (0 / 0 / 403) HTML/CSV