Make windows batch-scripts independent of calling directory

Change dir to batch-script location as first action. This
allows to run the script from outside the directory its stored.

build/workspaces/update-workspaces.bat

@@ -1,6 +1,7 @@
 @echo off
 rem ** Create Visual Studio Workspaces on Windows **
 
+cd /D "%~dp0"
 cd ..\bin
 if not exist ..\workspaces\vs2017\SKIP_PREMAKE_HERE premake5.exe --file="../premake/premake5.lua" --outpath="../workspaces/vs2017" %* vs2017
 cd ..\workspaces

libraries/get-windows-libs.bat

@@ -1,5 +1,7 @@
 rem **Download sources and binaries of libraries**
 
+cd /D "%~dp0"
+
 rem **SVN revision to checkout for windows-libs**
 rem **Update this line when you commit an update to windows-libs**
 set "svnrev=28243"