gitattributes: Do not export ast-grep infrastructure in source archives

Exclude infrastructure added by commit 61743471d9 (ast-grep: add a rule
to find adjacent string literals in cmStrCat calls, 2025-05-15,
v4.1.0-rc1~122^2~3).

.gitattributes

@@ -6,6 +6,7 @@
 .editorconfig    export-ignore
 .pre-commit-config.yaml export-ignore
 .typos.toml      export-ignore
+sgconfig.yml     export-ignore
 
 # Custom attribute to mark sources as using our C code style.
 [attr]our-c-style  whitespace=tab-in-indent  format.clang-format=18

Utilities/.gitattributes

@@ -1,3 +1,4 @@
+/ast-grep               export-ignore
 /Git                    export-ignore
 /GitSetup               export-ignore
 SetupForDevelopment.sh  export-ignore