Add depot_tools to env PATH

Fix for gclient not found, seems like sys.path.append does not work but path is added

build_ffmpeg.py

@@ -66,6 +66,9 @@ chromium_git = 'https://chromium.googlesource.com'
 os.system("git clone --depth=1 https://chromium.googlesource.com/chromium/tools/depot_tools.git")
 sys.path.append( os.getcwd() + "/depot_tools")
 
+#fix for gclient not found, seems like sys.path.append does not work but path is added
+os.environ["PATH"] += os.pathsep + os.getcwd() + "/depot_tools"
+
 if platform.system() == 'Windows' or 'CYGWIN_NT' in platform.system():
 	os.environ["DEPOT_TOOLS_WIN_TOOLCHAIN"] = '0'