Add C# SDK to nix dev shell

[chavic]

Agents and contributors using nix could not run the C# binding tests because dotnet was not provided by the dev shell. The C# test script generates bindings and then runs dotnet test, so nix should provide the SDK used by the repo.

• add the .NET 8 SDK to the default nix development shell
• document the nix command for running the C# binding test script

Closes #1555

Validation

• nix --extra-experimental-features "nix-command flakes" fmt -- --ci
• nix --extra-experimental-features "nix-command flakes" develop -c bash payjoin-ffi/csharp/contrib/test.sh

Disclosure: co-authored by Codex

[coveralls]

Coverage Report for CI Build 25926790907

Coverage remained the same at 85.293%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	13667
Covered Lines:	11657
Line Coverage:	85.29%
Coverage Strength:	395.85 hits per line

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💛 - Coveralls

[spacebear21]

concept ACK but @nothingmuch brought up out of band that dotnet-sdk_8 is considered insecure and furthermore we should set DOTNET_CLI_TELEMETRY_OPTOUT in the devshell in dotnet-sdk_10

[chavic]

concept ACK but @nothingmuch brought up out of band that dotnet-sdk_8 is considered insecure and furthermore we should set DOTNET_CLI_TELEMETRY_OPTOUT in the devshell in dotnet-sdk_10

I see, considering making dotnet-sdk_10 the default, the case for dotnet-sdk_8 was compatibility, hopefully 8 has faded out