fix(core): make 'key' parameter optional in register command (#30365)

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## Current Behavior
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Calling `nx register` requires users to enter a key.

## Expected Behavior
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Calling `nx register` should work without having users enter a key.

## Related Issue(s)
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merged. -->

Fixes #

Author: Cammisuli

packages/nx/src/command-line/register/command-object.ts

@@ -8,7 +8,7 @@ export interface RegisterOptions {
 }
 
 export const yargsRegisterCommand: CommandModule<{}, RegisterOptions> = {
-  command: 'register <key>',
+  command: 'register [key]',
   aliases: ['activate-powerpack'],
   describe: false,
   builder: (yargs) =>