pw_bloat: Don't put rebased paths in inputs

It's not needed to use rebase_path() for paths used by GN builtins - GN
always processes paths relative to the current build file or relative to
the GN --root.

Use of rebase_path should be limited to arguments passed to external
commands.

This was causing docs to fail to build due to putting build directory
relative paths in inputs (when it should be build file relative).

No-Docs-Update-Reason: bug fix
Change-Id: I994ddaaba44539360e693788bc80f311a7c44166
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48641
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Michael Spang <spang@google.com>