Support subninja chdir

[theoparis]

See ninja-build/ninja#1578

This solves the following issue:

subninja deps/musl/build.ninja

samu: file is missing and not created by any action: 'compat/time32/__xstat.c'

Example of the solution:

subninja build.ninja
  chdir = deps/musl

There does seem to be another issue with the upstream PR though. If you have a git clone step such as the following it will say there is a duplicate step with subninja chdir.

gitflags = --depth=1

rule clone
  command = git clone $gitflags $url $out

build deps/musl: clone
  url = https$://code.flowtr.dev/mirrors/musl