ci(gh-actions/install-nix): Enable binary substitution fallback

[PetarKirov]

Recently, we noticed both locally and in CI jobs errors like:

warning: error: unable to download
'https://blocksense.cachix.org/nar/be9530185b5e3d2f9fab587aa1bde52335bbd60b8e5efb79897a739577fdc482.nar.zst':
HTTP error 500 response body: Cannot serve NAR; retrying in 517 ms

which, in end the end result in:

error: some substitutes for the outputs of derivation
'/nix/store/xpmfjqgr0hdiwf8jiyqr7qs9f4v2r351-cargo-package-eth-keystore-0.5.0.drv'
failed (usually happens due to networking issues);
try '--fallback' to build derivation from source

To address this issue globally in the CI, we adjust the nix.conf file generated by metacraft-labs/nixos-modules/.github/install-nix GH action by:

• Enabling fallback for when binary substitution fails
• Reduce the number of retries from 5 to 2
• Set a connect-timeout to 30 seconds (before it was unlimited?!)
• Reduce narinfo-cache-negative-ttl from 3600 to 120 - allowing CI runners to check more often if something built by one machine is now available to others (via Cachix)

[github-actions]

Thanks for your Pull Request!

Below you will find a summary of the cachix status of each package, for each supported platform.

package	x86_64-linux	x86_64-darwin	aarch64-darwin
cachix	❌ build failed	❌ build failed	❌ build failed
dcd	✅ cached	🚫 not supported	🚫 not supported
dmd	✅ cached	🚫 not supported	🚫 not supported
dscanner	✅ cached	🚫 not supported	🚫 not supported
dub	✅ cached	✅ cached	✅ cached
folder-size-metrics	✅ cached	🚫 not supported	🚫 not supported
foundry	✅ cached	✅ cached	✅ cached
geth	✅ cached	🚫 not supported	✅ cached
ldc	✅ cached	🚫 not supported	🚫 not supported
lido-withdrawals-automation	✅ cached	✅ cached	✅ cached
mcl	✅ cached	🚫 not supported	🚫 not supported
mev-boost	✅ cached	🚫 not supported	🚫 not supported
nethermind	✅ cached	🚫 not supported	🚫 not supported
nimbus	✅ cached	🚫 not supported	🚫 not supported
nix	❌ build failed	❌ build failed	❌ build failed
nix-eval-jobs	❌ build failed	❌ build failed	❌ build failed
nix-fast-build	❌ build failed	❌ build failed	❌ build failed
pyroscope	✅ cached	✅ cached	✅ cached
random-alerts	✅ cached	✅ cached	✅ cached
rustToolchain	✅ cached	✅ cached	✅ cached
secret	✅ cached	🚫 not supported	✅ cached
serve-d	✅ cached	🚫 not supported	🚫 not supported
terraform	✅ cached	🚫 not supported	🚫 not supported
terranix	✅ cached	🚫 not supported	🚫 not supported
web3signer	✅ cached	🚫 not supported	🚫 not supported

[EmilIvanichkovv]

seems reasonable