[LLDB] Explicitly use python for version fixup (#144217)
On Windows, the post build command would open the script in the default
editor, since it doesn't know about shebangs. This effectively adds
`python3` in front of the command.
Amends https://github.com/llvm/llvm-project/pull/142871 /
https://github.com/llvm/llvm-project/pull/141116
