Update doc about cache-config.yaml

[jgp]

https://eth-cscs.github.io/stackinator/build-caches/#managing-keys says:

The cache-configuration would look like the following, where we assume that the cache is in $SCRATCH/uenv-cache:

root: $SCRATCH/uenv-cache
key: $HOME/.keys/spack-push-key.gpg

but env [...] make store.squashfs -j32:

/dev/shm/jg/stackinator.git/build/bwrap-mutable-root.sh 
--tmpfs ~ 
--bind build/tmp /tmp 
--bind build/store /user-environment spack gpg trust /users/piccinal/.keys/spack-push-key.gpg

will fail with:

==> Error: Command exited with status 2:
'/usr/bin/gpg2' '--with-colons' '/users/piccinal/.keys/spack-push-key.gpg'
gpg: can't open '/users/piccinal/.keys/spack-push-key.gpg'

A solution is to move the gpg key (and updating cache-config.yaml) out of $HOME.

[Ben Cumming]

Thanks.
The new documentation has been updated with instructions to not install you keys in HOME.