TUN-4911: Append Environment variable to Path instead of overwriting it

Environment Setting last specifies which part of the variable to append to. It was previously set to all which meant replace. (Ref: https://wixtoolset.org/documentation/manual/v3/xsd/wix/environment.html) .
(#432)

cloudflared.wxs

@@ -46,8 +46,8 @@
            Name="PATH"
            Value="[INSTALLDIR]."
            Permanent="no"
-           Part="all"
-           Action="set"
+           Part="last"
+           Action="create"
            System="yes" />
         </Component>
     </Directory>