Sporadic ccache fault

[fpjentzsch]

Every now and then we observe build flow failures during rtlsim. This seems to be more prevalent on our cluster with (slow) Lustre filesystem.

Running step: step_set_fifo_depths [14/20]
> /opt/conda/lib/python3.8/subprocess.py(1702)_execute_child()
-> raise child_exception_type(errno_num, err_msg, err_filename)
Build failed

/usr/local/share/verilator/include/verilated.mk:268: recipe for target 'verilated_threads.o' failed
/usr/local/share/verilator/include/verilated.mk:262: recipe for target 'Vfinn_design_wrapper.o' failed
ccache: error: /tmp/home_dir/.ccache/ccache.conf: No such file or directory
ccache: error: /tmp/home_dir/.ccache/ccache.conf: No such file or directory

I believe this might be caused by this race condition bug in ccache, which has been fixed in v3.4.3. FINN is using v3.4.1, so how do we go about updating this package? Is a switch from Ubuntu 18 to Ubuntu 20 for the Docker container planned? The latter supports v3.7.7 natively.