pin the rust toolchain to 1.61.0

1.61.0 in particular introduced new clippy lints that unexpectedly
failed CI until addressed. The lints are a bit tough to fix since
the toolchain action starts using new rust versions almost immediately
after release, so if you aren't using rustup, you may have a hard
time reproducing the lint results until your package manager updates
rust.

This brings an extra burden that we have to remember to make a
commit/PR to update rust in CI.

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