.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/selfupdate/ed25519.pem b/selfupdate/ed25519.pem
new file mode 100644
index 0000000..15dfa4b
--- /dev/null
+++ b/selfupdate/ed25519.pem
@@ -0,0 +1,3 @@
+-----BEGIN PUBLIC KEY-----
+MCowBQYDK2VwAyEAV4E+fJoShyziYCA5HjaafxIEX0DzIzwPwTyKOlappE8=
+-----END PUBLIC KEY-----