Fix passing arguments from backstage-cli backend:dev to package

The old Webpack version would invoke the backend package in the same
process, hence `process.argv` would contain the arguments passed to
`backstage-cli`. This no longer seems to be true in Webpack 5, so
arguments must be explicitly passed.

Signed-off-by: Joel Low <joel@joelsplace.sg>

.changeset/two-suits-act.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Fix `backstage-cli backend:dev` argument passing

packages/cli/src/lib/bundler/config.ts

@@ -318,6 +318,7 @@ export async function createBackendConfig(
       new RunScriptWebpackPlugin({
         name: 'main.js',
         nodeArgs: options.inspectEnabled ? ['--inspect'] : undefined,
+        args: process.argv.slice(3), // drop `node backstage-cli backend:dev`
       }),
       new webpack.HotModuleReplacementPlugin(),
       ...(checksEnabled