Auto-install .NET SDK in CI scripts when runner lacks it

Gitea runner build failed with "dotnet: command not found". Added
ensure_dotnet() to lib.sh, using the vendored dotnet-install.sh to
bootstrap the SDK into .dotnet/ when not already on PATH, called from
build.sh/test.sh/prepush.sh. Also trigger CI on build-runner-fix to
verify the fix.

scripts/ci/build.sh

@@ -6,6 +6,8 @@ set -euo pipefail
 cd "$(dirname "${BASH_SOURCE[0]}")" && source ./lib.sh
 cd "$CI_ROOT"
 
+ensure_dotnet
+
 log "Restoring and publishing MicCheck.Api (Release)"
 dotnet publish src/api/MicCheck.Api/MicCheck.Api.csproj -c Release