fix: use absolute path for nixos-version executable

Use /run/current-system/sw/bin/nixos-version instead of relying on
PATH, since the systemd service may not have the system binaries in
its PATH.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

collector/flake.go

@@ -226,11 +226,13 @@ func fetchFlakeMetadata(flakeURL string) (*flakeMetadata, error) {
 	return &data, nil
 }
 
+const nixosVersionPath = "/run/current-system/sw/bin/nixos-version"
+
 func getNixosVersionInfo() (*nixosVersionInfo, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 
-	cmd := exec.CommandContext(ctx, "nixos-version", "--json")
+	cmd := exec.CommandContext(ctx, nixosVersionPath, "--json")
 	output, err := cmd.Output()
 	if err != nil {
 		if exitErr, ok := err.(*exec.ExitError); ok {