gen-manifests: derrive seedArg from filename and env

To ensure that manifests get random(ish) and stable UUIDs we set
the rng seed arg based on the filename.

This should fix the issue discovered in
osbuild/manifest-db#124

that duplicated UUIDs for xfs/btrfs can trigger random(ish) and
hard to diagnose errors.

This is the same as
osbuild/osbuild-composer#4124
hopefully for the right place this time.

cmd/gen-manifests/main.go

@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"hash/fnv"
 	"io"
 	"os"
 	"path/filepath"
@@ -190,6 +191,14 @@ func makeManifestJob(
 	filename := fmt.Sprintf("%s-%s-%s-%s.json", u(distroName), u(archName), u(imgType.Name()), u(name))
 	cacheDir := filepath.Join(cacheRoot, archName+distribution.Name())
 
+	// ensure that each file has a unique seed based on filename
+	hash := func(s string) int64 {
+		h := fnv.New64()
+		h.Write([]byte(filename))
+		return int64(h.Sum64())
+	}
+	seedArg += hash(filename)
+
 	options := bc.Options
 
 	var bp blueprint.Blueprint