cmd/skipper: fix version and commit for go install

When skipper is installed via `go install` it has empty version and
commit values:
```
~$ go install github.com/zalando/skipper/cmd/skipper@latest
go: downloading github.com/zalando/skipper v0.21.2
~$ skipper -version
Skipper version  (commit: , runtime: go1.22.0)
```

Current build process of binaries specifies version and commit via `-ldflags`.

This change uses debug buildinfo to get version and commit if they are
not specified by `-ldflags` which is compatible with the current build process
but should also work for `go install` case.

If this change fixes `go install` case (which can only be tested after
package version is published) then we can add the same snippet
to other binaries and get rid of COMMIT_HASH ldflag.

We can not get rid of `-ldflags` fully though because `go build` does
not stamp version (see golang/go#50603).

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

cmd/skipper/main.go

@@ -17,7 +17,6 @@ package main
 import (
 	"fmt"
 	"runtime"
-
 	"runtime/debug"
 
 	log "github.com/sirupsen/logrus"
@@ -32,16 +31,16 @@ var (
 
 func init() {
 	if info, ok := debug.ReadBuildInfo(); ok {
+		if version == "" {
+			version = info.Main.Version
+		}
 		if commit == "" {
 			for _, setting := range info.Settings {
 				if setting.Key == "vcs.revision" {
 					commit = setting.Value[:min(6, len(setting.Value))]
 				}
 			}
 		}
-		if version == "" {
-			version = info.Main.Version
-		}
 	}
 }