Fix V1 legacy build: build via solution file and add workflow_dispatch ref input

[Copilot]

The V1 legacy build workflow was failing with Error NU5019: File not found: 'bin\Release\J2534-Sharp.dll' because building via the .csproj directly leaves $(SolutionDir) undefined — causing MSBuild to write output to the drive root (D:\bin\Release\) instead of <repo_root>\bin\Release\. The previous -BasePath Src fix was also incorrect for the same reason.

Changes

• Build via solution: nuget restore and msbuild now target J2534-Sharp.sln instead of src\J2534-Sharp.csproj, so $(SolutionDir) is set correctly and output lands in <repo_root>\bin\Release\ as the nuspec expects
• Remove -BasePath Src: no longer needed; bin\$configuration$\J2534-Sharp.dll in the nuspec resolves correctly from the repo root
• workflow_dispatch ref input: adds an optional ref input (default: V1-legacy) so the workflow can be dispatched from master to build the legacy branch using the fixed workflow file — working around the still-broken workflow on the V1-legacy branch itself
• Checkout uses dispatch ref: checkout step uses inputs.ref for manual dispatch, github.ref for tag-triggered runs
• Remove dead push trigger: push: branches: [V1-legacy] on master is a no-op — GitHub evaluates push triggers from the pushed branch's own workflow files, not master's

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