Mark cached_git_executable global

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 9ed3a88..90b4c55 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -743,6 +743,7 @@
 #               If true, the search is required to succeed, and the execution
 #               will terminate with sys.exit(1) if not found.
 def GIT(must_succeed=True):
+  global cached_git_executable
   if cached_git_executable is not None:
     return cached_git_executable
   # The order in the following is important, and specifies the preferred order