Alpine-based CI jobs fail with out-of-date Hackage index

Nightly Alpine jobs are failing with:

Running hadrian/build-cabal --flavour=release+fully_static -j8 --broken-test= --bignum=gmp --docs=none test:all_deps binary-dist -V...
Warning: Unknown/unsupported 'ghc' version detected (Cabal 3.6.2.0 supports
'ghc' version < 9.4): /opt/ghc/9.4.3/bin/ghc is version 9.4.3
Warning: Requested index-state 2023-10-05T11:38:51Z is newer than
'hackage.haskell.org'! Falling back to older state (2022-06-09T19:15:55Z).
Resolving dependencies...
cabal: Could not resolve dependencies:
...

For instance, see Job #1717907, Job #1717906, and Job #1717910.

This is strange since we earlier had run:

Running /usr/local/bin/cabal update -w /opt/ghc/9.4.3/bin/ghc hackage.haskell.org,2023-10-05T11:38:51Z...
Config file path source is default config file.
Config file /builds/ghc/ghc/cabal/config not found.
Writing default configuration to /builds/ghc/ghc/cabal/config
Downloading the latest package list from hackage.haskell.org
Updated package list of hackage.haskell.org to the index-state 2023-10-05T11:38:51Z

My initial suspicion was that we are using a different cabal-install between these two invocations. However, looking at ci.sh it appears that we use $CABAL consistently. Very odd.