Set up a few code owners (#40925)

Co-authored-by: Viral B. Shah <[email protected]>
(cherry picked from commit 3bbe22e)

Author: 2 people

.github/CODEOWNERS

@@ -1 +1,7 @@
-/.github/workflows/ @JuliaLang/github-actions
+/.github/ @JuliaLang/github-actions 
+/.buildkite/ @JuliaLang/github-actions
+/src/ @JuliaLang/compiler
+/stdlib/LinearAlgebra/ @JuliaLang/linearalgebra
+/stdlib/libblastrampoline_jll/ @staticfloat
+/test/ @JuliaLang/test
+