Update Missing CD in bats-core Install (#15363)

BUILD_macos.md

@@ -89,7 +89,8 @@ To run a full test suite locally, you will need
   tar xzvf bats.tar.gz
   sudo mkdir /usr/local/libexec
   sudo chown $USER /usr/local/libexec
-  ./install.sh /usr/local
+  cd bats-core-1.2.1
+  sudo ./install.sh /usr/local
   cd ../
   rm -rf bats-core-1.2.1 bats.tar.gz
   ```
@@ -111,4 +112,4 @@ To run a full test suite locally, you will need
   
   ```shell
   ulimit -n 2560 # 10x default
-  ```
+  ```