Use a relative path for the depfile's target to match Ninja's expectation; otherwise it thinks the target is always dirty.
TEST=build twice
Bug: https://github.com/flutter/flutter/issues/81074
Change-Id: I4cae7ab55f79b5206521c7090502c0769d2b5277
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/198443
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>