fix(install): support older awk checksum parsing

[fcoury-oai]

Why

The standalone installer validates package checksums with an awk interval expression. Older mawk releases do not support that expression, so they reject valid 64-character digests and report that the release manifest is missing an entry. This affects both x64 and ARM64 systems on common Debian-derived environments.

Fixes #24219.

What Changed

Replace the awk interval expression with an explicit length check plus rejection of non-hexadecimal characters. This preserves the existing SHA-256 validation and lowercase normalization while working with older awk implementations.

How to Test

1. Build and run the checksum predicate with mawk 1.3.4 20121129.
2. Confirm the old interval predicate rejects a valid 64-character digest.
3. Confirm the updated predicate accepts that digest.
4. Put the old mawk binary first on PATH as awk and run scripts/install/install.sh with an isolated HOME, CODEX_HOME, and CODEX_INSTALL_DIR.
5. Confirm Codex installs successfully and the installed binary reports version 0.140.0.
6. Verify the predicate rejects wrong-length digests, non-hexadecimal digests, and entries for another asset while accepting uppercase hexadecimal digests.