Bump to Lean v4.20.0-rc3

[dranov]

lean-auto currently doesn't work on macOS 15.4 due to a bug in Lean that has been fixed in v4.20.0-rc3.

This is the kind of error you see:

✖ [201/234] Building Auto.EvaluateAuto.EnvAnalysis
trace: .> LEAN_PATH=././.lake/build/lib/lean /Users/dranov/.elan/toolchains/leanprover--lean4---v4.18.0/bin/lean ././././Auto/EvaluateAuto/EnvAnalysis.lean -R ./././. -o ././.lake/build/lib/lean/Auto/EvaluateAuto/EnvAnalysis.olean -i ././.lake/build/lib/lean/Auto/EvaluateAuto/EnvAnalysis.ilean -c ././.lake/build/ir/Auto/EvaluateAuto/EnvAnalysis.c --plugin ././.lake/build/lib/lean/Auto_EvaluateAuto_NameArr.dylib --json
info: stderr:
libc++abi: terminating due to uncaught exception of type lean::exception: error loading library, dlopen(/Users/dranov/src/lean-auto-upstream/.lake/build/lib/lean/Auto_EvaluateAuto_NameArr.dylib, 0x0009): tried: '/Users/dranov/src/lean-auto-upstream/.lake/build/lib/lean/Auto_EvaluateAuto_NameArr.dylib' (__DATA_CONST segment missing SG_READ_ONLY flag), '/System/Volumes/Preboot/Cryptexes/OS/Users/dranov/src/lean-auto-upstream/.lake/build/lib/lean/Auto_EvaluateAuto_NameArr.dylib' (no such file), '/Users/dranov/src/lean-auto-upstream/.lake/build/lib/lean/Auto_EvaluateAuto_NameArr.dylib' (__DATA_CONST segment missing SG_READ_ONLY flag)

This PR bumps the toolchain to v4.20.0-rc3 so that users running macOS 15.4 can still build lean-auto.