internal/rosa: remove legacy exec helper

This change also adds a fourth stage to the bootstrap machinery, offering more correct toolchain validation that works around LLVM dynamic linking flaws.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/cmake.go

@@ -35,7 +35,7 @@ type CMakeHelper struct {
 var _ Helper = new(CMakeHelper)
 
 // extra returns the cmake handle alongside either ninja or make.
-func (attr *CMakeHelper) extra(int) P {
+func (attr *CMakeHelper) extra() P {
 	if attr != nil && attr.Make {
 		return P{_cmake, _make}
 	}