rustbuild: Improve assert about building tools once

In developing #61557 I noticed that there were two parts of our tools
that were rebuilt twice on CI. One was rustfmt fixed in #61557, but
another was Cargo. The actual fix for Cargo's double compile was
rust-lang/cargo#7010 and took some time to propagate here. In an effort
to continue to assert that Cargo is itself not compiled twice, I updated
the assertion in rustbuild at the time of working on #61557 but couldn't
land it because the fix wouldn't be ready until the next bootstrap.

The next bootstrap is now here, so the fix can now land! This does not
change the behavior of rustbuild but it is intended to catch the
previous iteration of compiling cargo twice. The main update here was to
consider more files than those in `$target/release/deps` but also
consider those in `$target/release`. That's where, for example,
`libcargo.rlib` shows up and it's the file we learn about, and that's
what we want to deduplicate.

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