Minor improvements

• When setting job parameter JENKINS_OVERRIDE_PIP_REQUIREMENTS, a local virtualenv is created on top of the requested conda environment and extended via pip install
• Keep job krb5 cache outside of /tmp and visible to the container (enables use of apptainer option --contain, see lhcb/MooreOnline#40 (comment 6919145))
• Use conda to set up the environment instead of manually manipulating PATH