fix(security): upgrade Go toolchain to 1.25.3 for CVE-2025-58187

Update Go toolchain to address HIGH severity vulnerability CVE-2025-58187
in the Go standard library.

Changes:
- flake.nix: Update from go_1_24 to go_1_25 (provides 1.25.3+)
- go.mod: Update toolchain directive from go1.24.0 to go1.25.3
- Maintain go 1.23.0 language compatibility

The vulnerability affects stdlib versions prior to 1.24.9 and 1.25.3.
This update ensures the project uses the patched toolchain version.

CVE Details:
- Package: stdlib
- Severity: HIGH
- Fixed Versions: 1.24.9, 1.25.3
- Link: CVE-2025-58187

Author: thpham

flake.nix

@@ -205,8 +205,8 @@
           packages =
             with pkgs;
             [
-              # Go toolchain
-              go_1_24
+              # Go toolchain (using 1.25.3+ for CVE-2025-58187 fix)
+              go_1_25
               gopls
               gotools
               go-tools

go.mod

@@ -1,8 +1,8 @@
 module github.com/thpham/gitlab-runner-kubevirt
 
-go 1.23.0
+go 1.23.0 // Uses Go 1.23 language features
 
-toolchain go1.24.0
+toolchain go1.25.3 // Builds with Go 1.25.3 compiler (has CVE fix)
 
 require (
 	barney.ci/shutil v0.1.0