[OSS] Fix build dir for check-doc-strings.sh (#35600)

`check-doc-strings.sh` got moved and is now in a `stdlib/scripts` directory
rather than a top-level `scripts` directory.  So, the behavior, when invoked,
would create a `build` directory inside `stdlib` rather than using the
top-level `build` directory where other build artifacts used.  This isn't
ideal.  Fix this by adjusting the `REPO_ROOT` path to go one directory back so
the warnings file produced lives in the top-level `build` directory.

modular-orig-commit: ec323fbe2abacb85fb3d628800c561c7d3ef16f1

stdlib/scripts/check-doc-strings.sh

@@ -8,7 +8,7 @@
 set -euo pipefail
 
 SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
-REPO_ROOT="${SCRIPT_DIR}/.."
+REPO_ROOT="${SCRIPT_DIR}/../.."
 
 check_doc_string() {
   local pkg=$1