The runner got its registration token through an `EnvironmentFile`
(`TOKEN=…`), so the daemon carried it for its whole life, and for a
`host` label act_runner copies its environment into every job step:
any workflow could read a token that registers runners on the forge.
The host unit now writes the bare token, and the runner unit loads it
with `LoadCredential=`. The upstream pre-start step, which registered
from `$TOKEN`, is replaced by one register step that reads the token
from `$CREDENTIALS_DIRECTORY` and runs `act_runner register` only when
`.runner` is missing, with the instance URL, name, labels and config
file upstream used. The precond now also removes `.runner` when its
labels differ from the configured ones, which upstream's step covered.
A changed token alone no longer re-registers.
Host jobs can still read `.runner` itself (#4979).
Closes#4974
0302b29ca4