Build the Go toolchain at genesis's base step, beside the base (hq issue 616)

Genesis's base step built only mesh-tools, the TypeScript toolchain. The store's and the bus's modules
carry Go bundles, compiled in mesh-tools-go, so genesis stopped at the store step: "this mesh holds no
copy of it. Build mesh-tools-go first". The base step now builds mesh-tools-go too, from
--tools-go-source or the repository beside --tools-source, and a test holds every toolchain a genesis
module's bundles compile in, read from the catalogue, to the ones the step builds.

internal/bootstrap/bootstrap.go

@@ -174,6 +174,10 @@ type Options struct {
 	// ToolsSource is where the shared base is built from. Everything with code of its own
 	// compiles against it, so it is the first thing the mesh builds for itself.
 	ToolsSource Source
+	// ToolsGoSource is where the Go toolchain is built from (`mesh-tools-go`, ADR 0142): every Go
+	// bundle compiles in it, and genesis's own modules have Go code (issue 616). Empty means the
+	// repository beside ToolsSource named mesh-tools-go, as the mesh keeps it.
+	ToolsGoSource Source
 	// CatalogSource is where the catalogue REPOSITORY is, for building its modules. The catalogue
 	// CHECKOUT (Catalogue, above) says what a module is; this is where a builder clones it.
 	CatalogSource Source