Packaging with read-only install directory crashes with `Permission denied`

[PetrKralCZ]

Running e.g. eb --read-only-installdir --package Perl-5.36.0-GCCcore-12.2.0-minimal.eb --rebuild --robot results in:

...
/kyukon/scratch/gent/vo/001/gvo00117/easybuild/RHEL8/cascadelake-ampere-ib/software/Ruby/3.2.2-GCCcore-12.2.0/lib/ruby/3.2.0/fileutils.rb:406:in `mkdir': Permission denied @ dir_s_mkdir - /tmp/vsc45304/eb-w1n5v7fj/eb-pkgs-uco9j300/package-dir-staging-9399f7f0048bd612711ffa79c70b0b18cda02b99f7fa35cb91c75fcfafa1/scratch/gent/vo/001/gvo00117/easybuild/RHEL8/cascadelake-ampere-ib/software/Perl/5.36.0-GCCcore-12.2.0-minimal/bin (Errno::EACCES)
...