Fixup paths to work with server-side Bash

This commit normalizes our Windows paths to use `/` instead of `\`.
These paths are compatible with both cmd and PowerShell, and are
required if the server-side shell is set to Bash.

The OpenSSH server executes all commands inside a default login shell
which cannot be controlled by the Vagrant configuration. So what ends up
getting executed on the server side looks something like this:

    "c:\\program files\\git\\bin\\bash.exe" -c "C:\\Windows\\Temp\\vagrant-ssh20200130-41670-1w5nsjy.bat"

By flipping the direction of the directory slashes, we end up with:

    "c:\\program files\\git\\bin\\bash.exe" -c "C:/Windows/Temp/vagrant-ssh20200130-43415-f1d5n2.bat"

This works whether the server-side shell is set to cmd, powershell, or
bash.

plugins/communicators/winssh/communicator.rb

@@ -45,7 +45,7 @@ module VagrantPlugins
           else
             tfile = Tempfile.new('vagrant-ssh')
             remote_ext = shell == "powershell" ? "ps1" : "bat"
-            remote_name = "#{machine_config_ssh.upload_directory}\\#{File.basename(tfile.path)}.#{remote_ext}"
+            remote_name = "#{machine_config_ssh.upload_directory}/#{File.basename(tfile.path)}.#{remote_ext}"
 
             if shell == "powershell"
               base_cmd = "powershell -File #{remote_name}"

plugins/communicators/winssh/config.rb

@@ -14,7 +14,7 @@ module VagrantPlugins
       def finalize!
         @shell = "cmd" if @shell == UNSET_VALUE
         @sudo_command = "%c" if @sudo_command == UNSET_VALUE
-        @upload_directory = "C:\\Windows\\Temp" if @upload_directory == UNSET_VALUE
+        @upload_directory = "C:/Windows/Temp" if @upload_directory == UNSET_VALUE
         if @export_command_template == UNSET_VALUE
           if @shell == "cmd"
             @export_command_template = 'set %ENV_KEY%="%ENV_VALUE%"'

plugins/provisioners/shell/provisioner.rb

@@ -40,12 +40,17 @@ module VagrantPlugins
 
       def upload_path
         if !defined?(@_upload_path)
-          @_upload_path = config.upload_path.to_s
+          case @machine.config.vm.guest
+          when :windows
+            @_upload_path = Vagrant::Util::Platform.unix_windows_path(config.upload_path.to_s)
+          else
+            @_upload_path = config.upload_path.to_s
+          end
 
           if @_upload_path.empty?
             case @machine.config.vm.guest
             when :windows
-              @_upload_path = "C:\\tmp\\vagrant-shell"
+              @_upload_path = "C:/tmp/vagrant-shell"
             else
               @_upload_path = "/tmp/vagrant-shell"
             end

test/unit/plugins/provisioners/shell/provisioner_test.rb

@@ -355,19 +355,21 @@ describe "Vagrant::Shell::Provisioner" do
           allow(machine).to receive_message_chain(:config, :vm, :guest).and_return(:windows)
         end
 
-        it "should default to C:\\tmp\\vagrant-shell" do
-          expect(vsp.upload_path).to eq("C:\\tmp\\vagrant-shell")
+        it "should default to C:/tmp/vagrant-shell" do
+          expect(vsp.upload_path).to eq("C:/tmp/vagrant-shell")
         end
       end
     end
 
     context "when upload_path is set" do
+      let(:upload_path) { "arbitrary" }
+
       let(:config) {
         double(
           :config,
           :args        => "doesn't matter",
           :env         => {},
-          :upload_path => "arbitrary",
+          :upload_path => upload_path,
           :remote?     => false,
           :path        => "doesn't matter",
           :inline      => "doesn't matter",
@@ -384,6 +386,18 @@ describe "Vagrant::Shell::Provisioner" do
       it "should use the value from from config" do
         expect(vsp.upload_path).to eq("arbitrary")
       end
+
+      context "windows" do
+        let(:upload_path) { "C:\\Windows\\Temp" }
+
+        before do
+          allow(machine).to receive_message_chain(:config, :vm, :guest).and_return(:windows)
+        end
+
+        it "should normalize the slashes" do
+          expect(vsp.upload_path).to eq("C:/Windows/Temp")
+        end
+      end
     end
 
     context "with cached value" do