Wallet: fix nostr receive

[callebtc]

Main thread used to exit too early but nostr threads kept running. Now the main thread will wait for a key press to exit.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 68.87%. Comparing base (be7a8a7) to head (8b612b2).

❗ Current head 8b612b2 differs from pull request most recent head 0961070. Consider uploading reports for the commit 0961070 to get more accurate results

Files	Patch %	Lines
cashu/wallet/cli/cli.py	0.00%	4 Missing ⚠️
cashu/wallet/nostr.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
- Coverage   68.91%   68.87%   -0.05%     
==========================================
  Files          74       74              
  Lines        6618     6622       +4     
==========================================
  Hits         4561     4561              
- Misses       2057     2061       +4     

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