--fetch fails for a git download when using a relative path for --sourcepath

[branfosj]

Noticed while testing for easybuilders/easybuild-easyconfigs#12347 - failure is not caused by recursive = True.

Using eb PyTorch-1.8.0-foss-2020b.eb --fetch --sourcepath abc fails with error

== 2021-03-25 09:59:22,577 run.py:225 INFO running cmd: tar cfvz abc/p/PyTorch/PyTorch-1.8.0.tar.gz --exclude .git pytorch
== 2021-03-25 09:59:22,712 build_log.py:169 ERROR EasyBuild crashed with an error (at easybuild/base/exceptions.py:124 in __init__): cmd "tar cfvz abc/p/PyTorch/PyTorch-1.8.0.tar.gz --exclude .git pytorch" exited with exit code -13 and output:
tar (child): abc/p/PyTorch/PyTorch-1.8.0.tar.gz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
pytorch/
pytorch/.bazelrc
pytorch/.bazelversion
pytorch/.circleci/
pytorch/.circleci/.gitignore
pytorch/.circleci/README.md
 (at easybuild/tools/run.py:537 in parse_cmd_output)

The directory abc/p/PyTorch/ is created - it looks like the tar command is being run from the wrong place - so cannot find abc/p/PyTorch/.

Whereas eb PyTorch-1.8.0-foss-2020b.eb --fetch --sourcepath /rds/bear-apps/devel/eb-sjb-up/src/easybuild-easyconfigs/easybuild/easyconfigs/tmp worked fine.