Update rust toolchain as a separately cacheable step

[hlinnaka]

Avoids repeatedly downloading the toolchain when iterating on compute parts locally, and probably helps on the CI too.

[jcgruenhage]

lgtm, thanks :)

[github-actions]

9053 tests run: 8404 passed, 0 failed, 649 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_sql_regress[v2-4]: debug-x86-64-without-lfc

Postgres 14

• test_replica_query_race: release-arm64-with-lfc

Code coverage* (full report)

• functions: 34.8% (8830 of 25357 functions)
• lines: 45.9% (71529 of 155850 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9e3a1cc at 2025-07-29T14:23:34.766Z :recycle:}