cd to / before running soong_build .

This lets one avoid any decisions as to when to chdir there during its
execution and leads to better sandboxing because the pwd doesn't leak to
init() functions anymore.

Test: Manual.
Change-Id: I1560da8ed3a621249426f9e8908aa890c21e13ba

shared/debug.go

@@ -0,0 +1,17 @@
+package shared
+
+import (
+	"os"
+	"os/exec"
+)
+
+// Finds the Delve binary to use. Either uses the SOONG_DELVE_PATH environment
+// variable or if that is unset, looks at $PATH.
+func ResolveDelveBinary() string {
+	result := os.Getenv("SOONG_DELVE_PATH")
+	if result == "" {
+		result, _ = exec.LookPath("dlv")
+	}
+
+	return result
+}