Revert "[build] Use an absolute path for the snapshot."

This reverts commit 269828ed34.

Reason for revert: This causes subsequent builds to be dirty no matter what.

Original change's description:
> [build] Use an absolute path for the snapshot.
>
> When constructing the command line for creating the kernel service
> snapshot, the variable `abs_output` actually contains a relative
> path, which means that copy/pasting the generated command line fails
> if not within the root build directory. This CL fixes that.
>
> Change-Id: I0d9fcafac489a6bd4771de1d6db1a2aa3623dcf8
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200861
> Reviewed-by: Daco Harkes <dacoharkes@google.com>
> Commit-Queue: Tess Strickland <sstrickl@google.com>

TBR=dacoharkes@google.com,sstrickl@google.com

Change-Id: I601d3c878d65ad6294d8f20c8b53d119b251ff43
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200940
Reviewed-by: Dan Field <dnfield@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

utils/application_snapshot.gni

@@ -145,7 +145,7 @@ template("_application_snapshot") {
     dfe = "$_dart_root/pkg/vm/bin/kernel_service.dart"
 
     abs_depfile = rebase_path(depfile)
-    abs_output = rebase_path(output)
+    abs_output = rebase_path(output, root_build_dir)
 
     vm_args = [
                 "--deterministic",