Alright, the workaround is to just run cabal update as a normal user.

This is worrisome, because it means that somehow, in some way, the build process (with both paru and yay) depends on some metadata files stored whoknowswhere in my home directory (which cabal downloads).

Not sure what the haskell-unordered-containers package does then; it looks like everything gets (re)-downloaded during the build process anyway. Perhaps it’s just meant for use with interactive Haskell environments or unpackaged “scripts”.

This doesn't build:

Warning: Requested index-state2021-03-23T15:08:40Z is newer than
'hackage.haskell.org'! Falling back to older state ().
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: cardano-addresses-3.4.0 (user goal)
[__1] unknown package: unordered-containers (dependency of cardano-addresses)
[__1] fail (backjumping, conflict set: cardano-addresses,
unordered-containers)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: cardano-addresses,
unordered-containers

And quite unsurprisingly, installing haskell-unordered-containers won’t solve the problem.

It looks like the Haskell building ecosystem is broken; not sure if that holds in general or if it’s an ArchLinux-specific problem.