env.bat.tmpl: Deal with blanks in OSGEO4W_ROOT (#859)

mswindows/osgeo4w/env.bat.tmpl

@@ -2,7 +2,7 @@ REM
 REM Environmental variables for GRASS OSGeo4W installer
 REM
 
-call %OSGEO4W_ROOT%\bin\py3_env.bat
+call "%OSGEO4W_ROOT%\bin\py3_env.bat"
 
 set GISBASE=%OSGEO4W_ROOT%\apps\grass\grass@POSTFIX@