ci: update Nix installation and caching

[YusukeShimizu]

Introduces caching of the Nix store using nix-community/cache-nix-action to avoide rate limit errors and speed up subsequent CI runs.

The GitHub API rate limit was exceeded, resulting in a CI process that used to take about 30 minutes being shortened to about 5 minutes.

[grubles]

Logs from https://github.com/ElementsProject/peerswap/actions/runs/10747587757/job/29810210862?pr=308:

Restoring a cache with the key "nix-Linux-5c8a7f8e191cbd7ffb1f785488ebdd34241da7e08b04c84a5ad22ad6cc307a5d".
Logs produced while restoring a cache.
  Received 197132288 of 2553320403 (7.7%), 187.8 MBs/sec
  Received 415236096 of 2553320403 (16.3%), 197.9 MBs/sec
  Received 629145600 of 2553320403 (24.6%), 199.9 MBs/sec
  Received 859832320 of 2553320403 (33.7%), 204.9 MBs/sec
  Received 1073741824 of 2553320403 (42.1%), 204.8 MBs/sec
  Received 1304428544 of 2553320403 (51.1%), 207.3 MBs/sec
  Received 1522532352 of 2553320403 (59.6%), 207.4 MBs/sec
  Received 1686110208 of 2553320403 (66.0%), 200.9 MBs/sec
  Received 1795162112 of 2553320403 (70.3%), 190.2 MBs/sec
  Received 2004877312 of 2553320403 (78.5%), 191.1 MBs/sec
  Received 2227175424 of 2553320403 (87.2%), 193.0 MBs/sec
  Received 2399141888 of 2553320403 (94.0%), 190.6 MBs/sec
  Received 2550136832 of 2553320403 (99.9%), 187.0 MBs/sec
  Cache Size: ~2435 MB (2553320403 B)
  /usr/bin/tar -xf /home/runner/work/_temp/0f2a4ef6-700c-40e6-ac1d-f0fc829cfa31/cache.tzst -P -C /home/runner/work/peerswap/peerswap --exclude-from /home/runner/work/_temp/d831f157-879f-4c6d-a90b-0024ebb763c1/nix-store-paths --use-compress-program unzstd
  Received 2553320403 of 2553320403 (100.0%), 173.9 MBs/sec
  Cache restored successfully

Looks good to me. ACK e304f66.