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

@@ -17,6 +17,7 @@ package main
 import (
 	"fmt"
 	"runtime"
+	"runtime/debug"
 
 	log "github.com/sirupsen/logrus"
 	"github.com/zalando/skipper"
@@ -28,6 +29,21 @@ var (
 	commit  string
 )
 
+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))]
+				}
+			}
+		}
+	}
+}
+
 func main() {
 	cfg := config.NewConfig()
 	if err := cfg.Parse(); err != nil {