chore: add suggestion for ntl switch to login cmd

[charliegroll]

Summary

Fixes #4462

If you're already logged in and try ntl login you aren't told about ntl switch, which may very well be what you mean to do.

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[github-actions]

📊 Benchmark results

Comparing with f14434c

Package size: 380 MB

(no change)

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Legend

• T-30 (f14434c): 380 MB
• T-29 (5036333): 380 MB
• T-28 (c8abe69): 380 MB
• T-27 (066841c): 380 MB
• T-26 (f87d9e6): 380 MB
• T-25 (88acdfd): 380 MB
• T-24 (03c34a4): 380 MB
• T-23 (3aa1fc6): 380 MB
• T-22 (e9c1789): 380 MB
• T-21 (fa32955): 380 MB
• T-20 (27366a4): 380 MB
• T-19 (92d2c74): 380 MB
• T-18 (48281ac): 380 MB
• T-17 (fc66595): 380 MB
• T-16 (a35b5b0): 380 MB
• T-15 (c13af13): 380 MB
• T-14 (4172001): 380 MB
• T-13 (f38d867): 380 MB
• T-12 (63c74f5): 380 MB
• T-11 (ca43189): 380 MB
• T-10 (9e4f88d): 380 MB
• T-9 (fa6995f): 380 MB
• T-8 (6096f96): 380 MB
• T-7 (c6ace29): 380 MB
• T-6 (8bcb8a5): 443 MB
• T-5 (0b2220a): 443 MB
• T-4 (16e054c): 443 MB
• T-3 (72d0400): 443 MB
• T-2 (71e1d8f): 443 MB
• T-1 (82c7377): 443 MB
• T (current commit): 380 MB

[melanierichards]

This looks great, thank you for jumping on it!