This runs build_tools/style.fish, which runs clang-format on C++, fish_indent on fish and (new) black on python.
If anything is wrong with the formatting, we should fix the tools, but automated formatting is worth it.
This switches fish to a "virtual" PWD, where it no longer uses getcwd to
discover its PWD but instead synthesizes it based on normalizing cd against
the $PWD variable.
Both pwd and $PWD contain the virtual path. pwd is taught about -P to
return the physical path, and -L the logical path (which is the default).
Fixes#3350
