[rust] Switch LTO flavor to "thin"

Bug: http://b/202032071

Thin LTO provides similar performance gains to full LTO but takes less
time during build.

Thin LTO also doesn't cause the error with `-C lto` and `-C
llvm-args=--runtime-counter-relocation` in b/202032071.

Test: presubmit
Test: verify b/202032071
Change-Id: Id5749aaea598d4b555a9d959ed753a312ae36f26