Setting the invalid CCACHE_ variables resulted in strange failure
in projects depending on meson, newer version like 0.55.3. For example
systemd build fails like the following errors:
```
* ACCESS DENIED: utimes: /mnt/host/source/ccache
* ACCESS DENIED: utimes: /mnt/host/source/ccache
F: utimes
S: deny
P: /mnt/host/source/ccache
A: /mnt/host/source/ccache
R: /mnt/host/source/ccache
C: ccache cc /build/amd64-usr/var/tmp/portage/sys-apps/systemd-246/work/systemd-246-abi_x86_64.amd64/meson-private/sanitycheckc.c -o /build/amd64-usr/var/tmp/portage/sys-apps/systemd-246/work/systemd-246-abi_x86_64.amd64/meson-private/sanitycheckc.exe -O1 -pipe -pipe -D_FILE_OFFSET_BITS=64
```
We should not set up ccache at all, as it has been already disabled in
coreos-overlay repo.
829cec45e8