internal/rosa/llvm: optionally writable source tree

This is required by the unit and regression tests.

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

internal/rosa/llvm.go

@@ -27,6 +27,9 @@ type llvmAttr struct {
 	script string
 	// Passed through to CMakeAttr.Prefix.
 	prefix *check.Absolute
+
+	// Passed through to CMakeAttr.Writable.
+	writable bool
 }
 
 const (
@@ -188,6 +191,8 @@ cp -r /system/include /usr/include && rm -rf /system/include
 			"ROSA_LLVM_RUNTIMES=" + strings.Join(runtimes, ";"),
 		}, attr.env),
 		ScriptEarly: scriptEarly, Script: script + attr.script,
+
+		Writable: attr.writable,
 	})
 }