go.mod: Keep compatibility constraint separate from build (#581)

go.mod

diff --git a/go.mod b/go.mod
index 86ea85f..0df4830 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,9 @@
 module github.com/hashicorp/terraform-exec
 
-go 1.25.8
+// Keep last digit at zero, use toolchain for Go build requirement
+go 1.25.0
+
+toolchain go1.25.8
 
 require (
 	github.com/google/go-cmp v0.7.0