Creates a step for mutating temporary directories created with addTempFiles.
Consider instead addWriteFiles which is for creating a cached directory
of files to operate on.
This should only be used with a tmp_path obtained via addTempFiles or
tmpPath.
pub fn addMutateFiles(b: *Build, tmp_path: LazyPath) *Step.WriteFile
pub fn addMutateFiles(b: *Build, tmp_path: LazyPath) *Step.WriteFile {
const wf = addWriteFiles(b);
wf.mode = .{ .mutate = tmp_path };
tmp_path.addStepDependencies(&wf.step);
return wf;
}