ignore /build instead of build/ in gitignore

this does two things:
1. allows making `build` a symlink (which is not considered a directory
   by git, thus removal of trailing `/`).
2. removes the need to special case `rustc_mir_build/src/build`
   (leading `/` makes git only ignore the `build` in the root)

.gitignore

@@ -46,8 +46,7 @@ no_llvm_build
 /inst/
 /llvm/
 /mingw-build/
-build/
-!/compiler/rustc_mir_build/src/build/
+/build
 /build-rust-analyzer/
 /dist/
 /unicode-downloads