Unable to verify PGP signature of plugin

Summary

I've used your ruleset template to create a plugin hosted in our private GitHub enterprise account. With a bit of tweaking, I've gotten the release workflow to correctly build and publish the binaries along with the checksums, with GPG being used, such that my release has the following assets:

checksums.txt
checksums.txt.sig
tflint-ruleset-custom_linux_amd64.zip
...
Source code (zip)
Source code (tar.gz)

I've then added an additional plugin {} block to .tflint.hcl along with the GPG public key, however when I run tflint --init it errors with Failed to install a plugin; Failed to check checksums.txt signature: openpgp: invalid argument: no armored data found.

If I don't set signing_key then the plugin installs but complains that I should ideally set signing_key.

I'm not sure what I'm doing wrong as I've pretty much followed your ruleset template repo. I've checked your aws plugin releases and your checksums.txt and checksums.txt.sig are of a similar format, I just don't have the checksums.txt.pem and checksums.txt.keyless.sig as I'm not using cosign which I presume creates these.

Command

GITHUB_TOKEN=... tflint --init

Terraform Configuration

n/a

TFLint Configuration

plugin "terraform" {
  enabled = true
  preset  = "all"
}

plugin "aws" {
  enabled = true
  version = "0.23.1"
  source  = "github.com/terraform-linters/tflint-ruleset-aws"
}

plugin "custom" {
  enabled = true

  version = "1.0.0"
  source  = "github.com/Org/tflint-ruleset-custom"

  signing_key = <<-KEY
  -----BEGIN PGP PUBLIC KEY BLOCK-----
  ...
  -----END PGP PUBLIC KEY BLOCK-----
  KEY
}

Output

Plugin `aws` is already installed
Installing `custom` plugin...
Failed to install a plugin; Failed to check checksums.txt signature: openpgp: invalid argument: no armored data found

TFLint Version

0.47.0

Terraform Version

No response

Operating System

[X] Linux
[ ] macOS
[ ] Windows

terraform-linters / tflint