Fix reference to build file with guaranteed build platforms.

[cipherself]

Description

refs #4261

This PR fixes a reference in README.md that referenced ci/build.sh which no longer exists, instead it now references ci/verify-build.sh

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @tgross35 (or someone else) some time within the next two weeks.

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[tgross35]

Thanks!