Fix GoReleaser build: use dir+main instead of file path

Using main: cmd/kvs/main.go caused go build to compile only that
single file as a standalone package, excluding serve.go and making
newServeCmd undefined. Switch to dir + main (.) so all .go files
in the package are included.

Author: skyoo2003

.goreleaser.yml

@@ -13,7 +13,8 @@ before:
 builds:
 - id: kvs
   binary: kvs
-  main: cmd/kvs/main.go
+  dir: cmd/kvs
+  main: .
   ldflags:
   - -s -w -X main.version={{ .Version }}
   goos: