chore: Update .gitignore (#3979)

- Un-ignore the `.github` folder
- Restrict the `build` exclude pattern to the repository root.

.gitignore

@@ -15,7 +15,7 @@ genConf
 x86_64-slc6-gcc48-opt
 x86_64-slc6-gcc49-opt
 .*
-build*
+/build*
 .asetup.save
 
 #json output
@@ -30,7 +30,7 @@ build*
 
 # dont ignore hidden configs
 !.clang-format
-!.github/**
+!.github
 !.gitignore
 !.gitlab-ci.yml
 !.clang-tidy