[ci] Roll pinned stable toolchain

[google-pr-creation-bot]

Automated changes by create-pull-request GitHub action

[joshlf]

My best guess as to what happened here: The way the auto-roller works is that it updates Cargo.toml to the latest stable version, re-generates the UI test .stderr files, and then sees if there's a git diff. Normally, there's only a git diff if there's a new stable toolchain version available. This is because the tests should be deterministic. However, as described in #706 (comment), we must be failing to run some UI tests, and so bad UI test .stderr files got committed. When this auto-roller ran, despite there not being any new stable version, some .stderr files were still updated, and so a git diff was detected, and so this PR was created.