cli: fix backend start hanging

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/wise-waves-approve.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Ignore `stdin` when spawning backend child process for the `start` command. Fixing an issue where backend startup would hang.

packages/cli/src/lib/experimental/startBackendExperimental.ts

@@ -95,7 +95,7 @@ export async function startBackendExperimental(options: BackendServeOptions) {
       process.execPath,
       [...loaderArgs, ...optionArgs, options.entry, ...userArgs],
       {
-        stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
+        stdio: ['ignore', 'inherit', 'inherit', 'ipc'],
         env: {
           ...process.env,
           BACKSTAGE_CLI_CHANNEL: '1',