Make $PATH available when building soong_build.

This is exclusively used by tests so it's awful, but Darwin builds are
currently broken so let's unbreak them first then start thinking.

Test: "m nothing" on Linux (I don't have access to a mac OS machine)
Change-Id: I1ea9ca71a5aff36a3454282a4c87ad368138baaa

ui/build/soong.go

@@ -171,8 +171,21 @@ func runSoong(ctx Context, config Config) {
 			"-f", filepath.Join(config.SoongOutDir(), file))
 
 		var ninjaEnv Environment
+
+		// This is currently how the command line to invoke soong_build finds the
+		// root of the source tree and the output root
 		ninjaEnv.Set("TOP", os.Getenv("TOP"))
 		ninjaEnv.Set("SOONG_OUTDIR", config.SoongOutDir())
+
+		// Needed for NonHermeticHostSystemTool() and that, only in tests. We should
+		// probably find a better way of running tests other than making $PATH
+		// available also to production builds. Note that this is not get same as
+		// os.Getenv("PATH"): config.Environment() contains the $PATH that redirects
+		// every binary through the path interposer.
+		configPath, _ := config.Environment().Get("PATH")
+		ninjaEnv.Set("PATH", configPath)
+
+		// For debugging
 		if os.Getenv("SOONG_DELVE") != "" {
 			// SOONG_DELVE is already in cmd.Environment
 			ninjaEnv.Set("SOONG_DELVE_PATH", shared.ResolveDelveBinary())