Use pip instead of python setup.py develop in CLI for plugin install

Apparently python setup.py develop tries to re-install all scripts
provided by plugins (maybe only under Windows?). Since that includes
OctoPrint's own executable, things will explode with an access denied
error there. Switching over to pip install -e . fixes that, and also
is more conform with the install approach the plugin will take in
production anyhow.

src/octoprint/cli/dev.py

@@ -204,7 +204,7 @@ class OctoPrintDevelCommands(click.MultiCommand):
 				click.echo("This doesn't look like an OctoPrint plugin folder")
 				sys.exit(1)
 
-			self.command_caller.call([sys.executable, "setup.py", "develop"], cwd=path)
+			self.command_caller.call([sys.executable, "-m", "pip", "install", "-e", "."], cwd=path)
 
 		return command