buildifier_test: fix FINDSTR: Cannot open MANIFEST

This happens on Windows when using `buildifier_test`:
```
INFO: Running command line: 'external\bazel_tools\tools\test\tw.exe' buildifier/test.bat
Executing tests from //buildifier:test
-----------------------------------------------------------------------------
FINDSTR: Cannot open MANIFEST
The expression after '&' in a pipeline element produced an object that was not valid. It must result in a command
name, a script block, or a CommandInfo object.
At line:1 char:893
+ ...  (Should-Exclude $_.Path) }; foreach ($File in $Files) { & '' -mode=d ...
+                                                                ~~
    + CategoryInfo          : InvalidOperation: (:String) [], RuntimeException
    + FullyQualifiedErrorId : BadExpression
```

The present change simply consists in giving a second try with
`$RUNFILES_MANIFEST_FILE` and failing should both be absent.

Author: rdesgroppes

buildifier/runner.bat.template

@@ -9,7 +9,15 @@ rem Unquote the arguments
 set stripped_args=%stripped_args:'=%
 
 rem Get the absolute path to the buildifier executable
-for /f "tokens=2" %%i in ('findstr /r "\<buildifier\.exe\>" MANIFEST') do (set buildifier_abs_path=%%i)
+if exist MANIFEST (
+    set manifest_file=MANIFEST
+) else if exist "%RUNFILES_MANIFEST_FILE%" (
+    set "manifest_file=%RUNFILES_MANIFEST_FILE:/=\%"
+) else (
+    >&2 echo Error: manifest file not found
+    exit /b 1
+)
+for /f "tokens=2" %%i in ('findstr /r "\<buildifier\.exe\>" "%manifest_file%"') do (set buildifier_abs_path=%%i)
 
 powershell ^
 function Buildify($Root)^