[windows-1809] fix vcpkg git clone dst

windows-1809/base/Dockerfile

@@ -46,7 +46,7 @@ RUN [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tl
     [Environment]::SetEnvironmentVariable('Pylon5_DIR', 'C:/ProgramData/Pylon5/Development', [EnvironmentVariableTarget]::Machine);
 
 RUN $vcpkg_dir = 'C:/ProgramData/vcpkg'; \
-    git clone -b robofish --single-branch https://git.imp.fu-berlin.de/bioroboticslab/robofish/vcpkg.git; \
+    git clone -b robofish --single-branch https://git.imp.fu-berlin.de/bioroboticslab/robofish/vcpkg.git $vcpkg_dir; \
     cd $vcpkg_dir; \
     git checkout c6f4350fcbec06f8b9f2c22daeb9c651c9e28605; \
     ./scripts/bootstrap.ps1 -disableMetrics; \