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/python.go

@@ -26,7 +26,7 @@ type PipHelper struct {
 var _ Helper = new(PipHelper)
 
 // extra returns python, or pytest if defaults are assumed.
-func (attr *PipHelper) extra(int) P {
+func (attr *PipHelper) extra() P {
 	if attr == nil || (!attr.SkipCheck && attr.Check == "") {
 		return P{_pythonPyTest}
 	}