$in, $out, $in_newline don't work in variables set on build edges (only on rules)

[nico]

This is because $in & co are evaluated by an EdgeEnv, and when ParseEdge evaluates the bindings for a build edge, it doesn't build an EdgeEnv.

My use case is something like this, where commands are always set on build edges, never intentionally on a rule:

rule do
  command = exit 1
build array_type_info.o: do array_type_info.cc
  description = armeabi C++: src/array_type_info.cc
  command = clang++ -c -o array_type_info.o array_type_info.cc
# Many more build lines like this

This is for a system that currently generates makefiles, and where it's hard to know rules in advance, so not really using rules makes things easier. I'd like to say $in and $out in the command of the build, so that input and output aren't repeated. (In practice, listing them twice seems to work well, even if the outputs are very long lists like they are for link lines. But it'd be nice if this worked for self-consistency too.)