ci: fix goreleaser config

Currently goreleaser generated assets end up at URLs like:

   .../v0.3.0-beta.5/cue_0.3.0-beta.5_Linux_x86_64.tar.gz

Note the inconsistency between "v0.3.0-beta.5" and "0.3.0-beta.5"
(missing "v"), not to mention the use of mapped versions of GOOS and
GOARCH.

Fix this so that unity and others can rely on a more consistent download
URL format.

Tested locally that the right file names are generated.

Change-Id: Id4dacf7132af1a84d76ee3ac9a41540f7b2c06c5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8821
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

.goreleaser.yml

@@ -26,12 +26,8 @@ builds:
     - arm64
 
 archives:
-- replacements:
-    darwin: Darwin
-    linux: Linux
-    windows: Windows
-    386: i386
-    amd64: x86_64
+- name_template: "{{ .ProjectName }}_{{ .Tag }}_{{ .Os }}_{{ .Arch }}{{ if .Arm }}v{{ .Arm }}{{ end }}{{ if .Mips }}_{{ .Mips }}{{ end }}"
+  replacements:
   files:
     - LICENSE
     - README.md