Windows local build fixes

* The directory dcos_build_venv must be deleted only if it already exists.
* Fix pip upgrade. The following issue explains the situation and the
  proposed workaround proposed: pypa/pip#3804

build_local_windows.ps1

@@ -14,7 +14,10 @@ if ( ! $tmpdir ) {
 }
 
 # Cleanup from previous build
-rm -recurse "$tmpdir/dcos_build_venv"
+$venvDir = "$tmpdir/dcos_build_venv"
+if(Test-Path $venvDir) {
+    rm -recurse -force "$tmpdir/dcos_build_venv"
+}
 
 # Force Python stdout/err to be unbuffered.
 $env:PYTHONUNBUFFERED="notempty"

prep_local_windows.ps1

@@ -10,7 +10,7 @@ if (!$env:VIRTUAL_ENV) {
 }
 
 # Install the latest version of pip
-pip install -U pip
+python -m pip install -U pip
 
 # Install the packages in editable mode to the virtual environment.
 pip install -e $PWD