host-modules: atomic_write_secret takes the value as an argument, not stdin

The pipe contract had a gap: if a producer piped into
atomic_write_secret exited non-zero after writing partial output,
cat still saw a clean EOF and wrote that partial content through to
the live target via mv — pipefail only reported the failure
afterward, once the bad write was already committed. The helper now
takes the value as its 4th argument and writes it itself with printf
(a shell builtin, so the value never touches an external process's
own argv/environ, same as a function argument never does), so there
is no pipe left to fail silently.

Callers that compute the value with a command now capture it into a
variable first (`value=$(cmd)`), which fails under `set -e` before
atomic_write_secret is ever called — swarm-bao.nix's pin.env site is
the one that needed this (`pin_env_value="BAO_HSM_PIN=$(cat ...)"`).
All seven call sites converted; output is byte-identical (same
printf '%s\n' framing, now applied inside the helper instead of by
each caller).

Refs #4723

nix/host-modules/swarm-grafana.nix

@@ -693,7 +693,7 @@ in
         # grants the group nothing; if this mode ever widens, the gid has to be
         # discovered at runtime rather than assumed.
         install -d -m 0755 ${lib.escapeShellArg hostSecretDir}
-        printf '%s\n' "$secret" | atomic_write_secret 0400 ${lib.escapeShellArg "${toString config.ids.uids.grafana}:0"} ${lib.escapeShellArg hostSecretPath}
+        atomic_write_secret 0400 ${lib.escapeShellArg "${toString config.ids.uids.grafana}:0"} ${lib.escapeShellArg hostSecretPath} "$secret"
       '';
     };