expr: bracket expression mismatches on Windows arm64

[sylvestre]

On aarch64-pc-windows-msvc only:

$ coreutils expr '[^a]' : '\[^a]'
0        # exit 1

Expected 4 (exit 0): \[ is a literal [, so the pattern matches the whole string. This passes on Linux, macOS and x86_64-pc-windows-msvc -only the Windows arm64 build differs.

Found when the Windows arm64 CI job started building and testing natively in #14246. test_expr::test_regex_caret is #[ignore]d on that target for now; the ignore should be removed once this is fixed.

[oech3]

@wtcpython Is this issue still active with fancy-regex?