fix(install): support older awk checksum parsing (#28784)

## 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.

scripts/install/install.sh

@@ -213,7 +213,7 @@ package_archive_digest() {
   manifest_path="$2"
 
   digest="$(awk -v asset="$asset" '
-    $2 == asset && $1 ~ /^[0-9a-fA-F]{64}$/ {
+    $2 == asset && length($1) == 64 && $1 !~ /[^0-9a-fA-F]/ {
       print tolower($1)
       found = 1
       exit