Skip clang updates on Windows

We do not use clang to build on Windows, so we should skip trying to
update clang.  This is critical to MSVS 2019 support, since the older
clang tools we depend on do not recognize that newer environment.
Since we don't need clang anyway, skipping the update avoids useless
errors about finding a matching clang version for the environment.

Issue #867 (MSVS 2019)
Issue #336 (progress toward replacing Travis & Appveyor with GitHub
  Actions, which uses MSVS 2019)
b/190743862 (internal; tracking replacement of Travis)

Change-Id: I5600ed809b11e68444034a06cc891403e6bfb5cc

DEPS

@@ -111,6 +111,8 @@ hooks = [
   {
     # Pull clang if needed or requested via GYP_DEFINES (GYP_DEFINES="clang=1").
     "name": "clang",
+    # Skip clang updates on Windows, where we don't use clang.
+    "condition": "not checkout_win",
     "pattern": ".",
     "action": ["python", "src/packager/tools/clang/scripts/update.py", "--if-needed"],
   },