.goreleaser.yml: add binary-only releases and ed25519 signatures

This lays the groundwork for future secure self-updates.
The corresponding public key is in selfupdate/ed25519.pem.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s5b7440b1a9e2c5e3k
diff --git a/.goreleaser.yml b/.goreleaser.yml
index 71b255a..ddc4407 100644
--- a/.goreleaser.yml
+++ b/.goreleaser.yml
@@ -36,6 +36,19 @@
     files:
       - README.md
       - LICENSE
+  - id: releaseBinary
+    format: binary
+    name_template: "{{ .ProjectName }}_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
+
+signs:
+  - id: ed25519-sign
+    cmd: selfupdatectl
+    args:
+      - "sign"
+      - "--private-key={{.Env.SELFUPDATE_PRIVKEY_PATH}}"
+      - "${artifact}"
+    signature: "${artifact}.ed25519"
+    artifacts: all
 
 release:
   github: