ci bash script relies on GNU-only \b regex syntax

[jiengup]

Bug description

Follow-up to #3694.

scripts/ci/sync-rustc-version.sh relies on \b as a word-boundary assertion in RUST_IMAGE_PATTERN (line 82) and in the grep/sed expressions at lines 111, 113, 122, and 124. \b is GNU regex behavior rather than a portable sed/grep construct; implementations without it interpret this as brust: and never match FROM rust:....

The script therefore only works correctly with a GNU-compatible toolchain: --check can produce a false green and --fix can be a silent no-op for FROM-form Dockerfiles, while ARG RUST_VERSION remains handled. Replace \b with a portable expression.

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