Fix re_types_builder falsely triggering rebuilds

[teh-cmc]

#3460 removed committed hashes, but re_types_builder is still subscribed to changes to source_hash.txt.

Even though the build script will early exit as soon as it realizes that the hash is None, this is enough to invalidate rustc's cache, which then invalidates every crate downstream.

main:

this branch:

Fixes:

• The roundtrip test suite execution time regression
• Fix unnecessary rebuilds by updating to maturin 1.3.1 #3266

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• I have tested demo.rerun.io (if applicable)

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[teh-cmc]

Reverting #3460 does what we'd expect: