Add missing build_script_inputs for test_rlib

The build script for this test library is unusual in that, at runtime,
it refers to build.rs. (Normally build.rs is used only to _build_ the
build script rather than at runtime.)

Bug: 346899819
Cq-Include-Trybots: luci.chromium.try:android-rust-arm32-rel,android-rust-arm64-dbg,android-rust-arm64-rel,linux-rust-x64-dbg,linux-rust-x64-rel,mac-rust-x64-dbg,win-rust-x64-dbg,win-rust-x64-rel
Change-Id: I4aa687ab9cc8afde41bc1b9ce7eedfaa9ea35a7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5626194
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Cr-Commit-Position: refs/heads/main@{#1315647}