Find the .NET SDK in ~/.dotnet when it is not on PATH

[Antaris]

bld self-update failed with dotnet was not found on PATH after a script install.

scripts/install.sh puts the SDK in ~/.dotnet and only adds that directory to PATH for the install session. The self-contained bld binary stays on PATH via ~/.local/bin, so later self-update / build runs often cannot see dotnet.

Change

• Resolve the SDK muxer from PATH, then ~/.dotnet, then well-known system locations
• Require an sdk directory so a runtime-only install is skipped
• Point child dotnet processes at that SDK (DOTNET_ROOT / DOTNET_HOST_PATH) so a self-contained bld does not hide it
• Same lookup is used by build

# reproduces before this change
env PATH="$HOME/.local/bin:/usr/bin:/bin" bld self-update

  

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