replace BASE_BOX with FROM_SCRATCH, add PREBUILT_VERSION, and add small troubleshooting tip for NO_SUBMISSIONS=1 for git troubles

Author: IDzyre

_docs/developer/getting_started/vm_install_using_vagrant.md

@@ -228,11 +228,20 @@ Below are quick steps to get everything installed and running.
 
    If you wish to run `vagrant up` from scratch, on Linux or Mac type:
    ```
-   BASE_BOX=1 vagrant up
+   FROM_SCRATCH=1 vagrant up
    ```
    or on Windows, type: 
    ```
-   SET BASE_BOX=1
+   SET FROM_SCRATCH=1
+   vagrant up
+   ```
+   If you wish to use a specific version of the pre-packaged Submitty VM, on Linux or Mac type:
+   ```
+   PREBUILT_VERSION={version} vagrant up
+   ```
+   or on Windows, type: 
+   ```
+   SET PREBUILT_VERSION={version}
    vagrant up
    ```
    This process will take anywhere from 30 minutes to a few hours depending on your

_docs/developer/troubleshooting/installation_troubleshooting.md

@@ -44,6 +44,17 @@ with no explanation, then there are a couple of things that may be going wrong:
    This will delete all virtual machine settings. Then install
    the latest version of Virtual Box and vagrant from the links given in step 3 (using Ubuntu Software).
 
+* If you are not using the prebuilt Submitty VM, and you get an error like
+  ```
+  FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpad2cfj3v/abernl'
+  ```
+  This is a known issue with our implementation of Git gradeables, which will be resolved when we have the time. Until then, you can do.
+  ```
+  NO_SUBMISSIONS=1 vagrant up
+  ```
+
+  __This will not have submissions in the VM, so you will have to manually add some__
+
 * If it has been a while since your last `vagrant destroy` and
     `vagrant up` you may need to update/upgrade/reinstall the virtual
     box, vagrant, and the installed boxes on your