ci: take the AUR key base64-encoded so the runner masks it [skip ci]

README.md

@@ -87,7 +87,9 @@ source in the `PKGBUILD` (and drop the `-bin` suffix), or don't package it.
    public key to it (AUR account settings).
 2. Add the matching **private** key as a repository secret named
    `AUR_SSH_PRIVATE_KEY`
-   (Settings → Actions → Secrets → Add secret).
+   (Settings → Actions → Secrets → Add secret), **base64-encoded on one line**
+   (`base64 -w0 < key`). The Gitea runner prints each step's environment and
+   does not mask a multi-line secret, so a plain key would end up in the log.
 3. Optionally set the repository variable `AUR_GIT_NAME` and the repository
    **secret** `AUR_GIT_EMAIL` to control the commit identity used on the AUR
    (defaults: `Felitendo` / the maintainer's old GitHub noreply address).

scripts/update-package.sh

@@ -183,7 +183,16 @@ fi
 # container $HOME and the passwd home directory disagree, and ssh resolves
 # "~" through the latter, silently ignoring anything written to $HOME/.ssh.
 sshdir="$(mktemp -d)"
-printf '%s\n' "$AUR_SSH_PRIVATE_KEY" > "$sshdir/key"
+# The secret holds the key base64-encoded on one line. The Gitea runner prints
+# each step's environment and only masks a secret it finds there word for
+# word, but it prints a multi-line value with escaped newlines: a plain key
+# would show up in the log in full.
+if [[ "$AUR_SSH_PRIVATE_KEY" == -----BEGIN* ]]; then
+  echo "::error::$pkg: AUR_SSH_PRIVATE_KEY must be base64-encoded on one line" \
+       "(base64 -w0), or the runner prints it in the log."
+  exit 1
+fi
+base64 -d <<< "$AUR_SSH_PRIVATE_KEY" > "$sshdir/key"
 chmod 600 "$sshdir/key"
 # Pinned host key, see https://aur.archlinux.org
 echo 'aur.archlinux.org ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEuBKrPzbawxA/k2g6NcyV5jmqwJ2s+zpgZGZ7tpLIcN' \