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/paths.go

@@ -30,6 +30,21 @@ type SharedPaths interface {
 	BazelMetricsDir() string
 }
 
+// Joins the path strings in the argument list, taking absolute paths into
+// account. That is, if one of the strings is an absolute path, the ones before
+// are ignored.
+func JoinPath(base string, rest ...string) string {
+	result := base
+	for _, next := range rest {
+		if filepath.IsAbs(next) {
+			result = next
+		} else {
+			result = filepath.Join(result, next)
+		}
+	}
+	return result
+}
+
 // Given the out directory, returns the root of the temp directory (to be cleared at the start of each execution of Soong)
 func TempDirForOutDir(outDir string) (tempPath string) {
 	return filepath.Join(outDir, ".temp")