fix github actions runs on forks

This commit contains two separate fixes:

1) use lowercased github username/repository for the docker
repository name.
This is because image registries only accept
lowercase in image repository names, but github stores the
repository and username in a case-sensitive manner. This broke
image uploads for me, as my username has uppercase chars.

2) change run conditions for some steps.
It will no longer attempt to set up SSH web publishing if the SSH
private key is not set.
It will also run the image registry upload steps if registry
usernames are missing, instead skipping individual uploads where
the token for that registry is missing.

Finally, it simplifies the sccache run conditions to use the
`SCCACHE_GHA_ENABLED` env variable, rather than duplicating that
logic.

Signed-off-by: Jade Ellis <[email protected]>