Update go toolchain directive to v1.26.4

Author: renovate[bot]

go.mod

@@ -2,7 +2,7 @@ module golift.io/cnfg
 
 go 1.24
 
-toolchain go1.26.0
+toolchain go1.26.4
 
 // for tests.
 require github.com/stretchr/testify v1.11.1