### Description
This PR introduces GPG signing for all Coder *slim-binaries*.
Detached signatures will allow users to verify the integrity and
authenticity of the binaries they download.
### Changes
* `scripts/sign_with_gpg.sh`: New script to sign a given binary
using GPG. It imports the release key, signs the binary, and
verifies the signature.
* `scripts/build_go.sh`: Updated to call `sign_with_gpg.sh` when the
`CODER_SIGN_GPG` environment variable is set to 1.
* `.github/workflows/release.yaml`: The` CODER_SIGN_GPG` environment
variable is now set to 1 during the release build, enabling GPG
signing for all release binaries.
* `.github/workflows/ci.yaml`: The `CODER_SIGN_GPG` environment
variable is now set to 1 during the CI build, enabling GPG
signing for all CI binaries.
* `Makefile`: Detached signatures are moved to the `/site/out/bin/
`directory