[llvm] [lit] Deprecate execute_external=True in ShTest (PR #201732)

[via llvm-commits]

dyung wrote:

> LLVM Buildbot has detected a new failure on builder `sanitizer-aarch64-linux` running on `sanitizer-buildbot7` while building `llvm` at step 2 "annotate".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/51/builds/38295
> 
> Here is the relevant piece of the build log for the reference

One of my bots is also failing with the exact same error: https://lab.llvm.org/buildbot/#/builders/174/builds/36815

https://github.com/llvm/llvm-project/pull/201732