Use armored keyring for APT repository

[kenhys]

Since apt 2.9.16, it rejects the keybox format files. So we should use
the armored format instead.

ref. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1088656#35

> The only format which is GnuPG specific, are for example its internal
> keybox keyrings, which I don't think has ever been used as any kind
> of public interchange format.

In practical use case, Debian 13 (trixie) or Ubuntu 25.04 will be
affected. Without it, it causes verification error with apt-get update.

[kenhys]

Steps to verify:

• apt install -y vim curl sudo
• execute install script which is documented on fluent.org
• change /etc/apt/sources.list.d/fluent.sources or fluent-lts.sources, point to test/experimental
• apt update
• apt upgrade -y, fluent-apt-source or fluent-lts-apt-source should be updated.
• apt update should be succeed

Result:

• bullseye v5 OK
• bullseye lts OK
• bookworm v5 OK
• bookworm lts NG (fluent-package depends libssl1.1, it should be fixed as another issue)
• focal v5 OK
• focal lts OK
• jammy v5 OK
• jammy lts OK
• noble v5 OK
• noble lts OK

NOTE: tested with docker.io/debian/bookworm

[Watson1978]

👍🏻

[daipom]

LGTM. Thanks!