Appends the contents of arg, verbatim, to the command line that will be
passed to the process being run.
If arg is an input file, addFileInput (or related function) must be
used instead to ensure correct cache behavior.
If arg is an output file, addOutputFileArg (or related function) must
be used instead to ensure correct cache behavior.
pub fn addArg(run: *Run, arg: []const u8) void
pub fn addArg(run: *Run, arg: []const u8) void {
const graph = run.step.owner.graph;
const arena = graph.arena;
run.argv.append(arena, .{ .bytes = graph.dupeString(arg) }) catch @panic("OOM");
}