xtensa-build-zephyr.py: add cavstool.py to staging directory

[marc-hb]

2 commits. Main one:

xtensa-build-zephyr.py: add cavstool.py to staging directory

This is required to read Zephyr logs.

[marc-hb]

At the very bottom of https://github.com/thesofproject/sof/runs/5877350511:

build-sof-staging
├── tools
│   ├── cavstool.py
│   └── sof-logger
└── sof
    ├── sof-icl.ldc
    ├── sof-imx8x.ldc
    ├── sof-imx8m.ldc
    ├── sof-cnl.ldc

Same thing at https://sof-ci.01.org/sofpr/PR5667/build12665/build/