It's better to avoid references to this global variable, but, in the
cases where it's needed, such as in std.debug.print and collecting stack
traces, better to share the same instance.
File arguments added to `std.Build.Step.Run` with e.g. `addFileArg` are
not necessarily passed as absolute paths. It used to be the case that
they were as a consequence of an unnecessary path conversion done by the
frontend, but this no longer happens, at least not always, so these
tests were sometimes failing when run locally. Therefore, the standalone
tests must handle cwd-relative CLI paths correctly.
Andrew Kelley