Haskell.nix cannot build stackage packages

[andreabedini]

E.g.

$ nix-build -A 'pkgs-unstable.haskell-nix.snapshots."lts-20.10".tasty-discover'

fails with

building '/nix/store/xw1lbyk17ywd4yj2724ib5m7ljikwf4a-unix-lib-unix-2.7.2.2.drv'...
unpacking sources
unpacking source archive /nix/store/6fv3psnba96vr26xgd1dzv40vbmmwn9m-unix-2.7.2.2.tar.gz
source root is unix-2.7.2.2
setting SOURCE_DATE_EPOCH to timestamp 1500501876 of file unix-2.7.2.2/unix.cabal
patching sources
configuring
Configure flags:
--prefix=/nix/store/ivyzwrga2gv4qxgg31q2waw5s708fik9-unix-lib-unix-2.7.2.2 lib:unix --package-db=clear --package-db=/nix/store/jbns7dp118zp5df1wknpp40pcinp9y3p-unix-lib-unix-2.7.2.2-config/lib/ghc-9.2.5/package.conf.d --with-ghc=ghc --with-ghc-pkg=ghc-pkg --with-hsc2hs=hsc2hs --with-gcc=cc --with-ld=ld.gold --ghc-option=-optl-fuse-ld=gold --ld-option=-fuse-ld=gold --with-ar=ar --with-strip=strip --disable-executable-stripping --disable-library-stripping --disable-library-profiling --disable-profiling --enable-static --enable-shared --disable-coverage --enable-library-for-ghci --datadir=/nix/store/p607d39a632lsp87wc21mckrcijb47ch-unix-lib-unix-2.7.2.2-data/share/ghc-9.2.5 --enable-split-sections 
Configuring library for unix-2.7.2.2..
Setup: Encountered missing or private dependencies:
base >=4.5 && <4.11,
bytestring >=0.9.2 && <0.11,
time >=1.2 && <1.9

error: builder for '/nix/store/xw1lbyk17ywd4yj2724ib5m7ljikwf4a-unix-lib-unix-2.7.2.2.drv' failed with exit code 1;
       last 10 log lines:
       > patching sources
       > configuring
       > Configure flags:
       > --prefix=/nix/store/ivyzwrga2gv4qxgg31q2waw5s708fik9-unix-lib-unix-2.7.2.2 lib:unix --package-db=clear --package-db=/nix/store/jbns7dp118zp5df1wknpp40pcinp9y3p-unix-lib-unix-2.7.2.2-config/lib/ghc-9.2.5/package.conf.d --with-ghc=ghc --with-ghc-pkg=ghc-pkg --with-hsc2hs=hsc2hs --with-gcc=cc --with-ld=ld.gold --ghc-option=-optl-fuse-ld=gold --ld-option=-fuse-ld=gold --with-ar=ar --with-strip=strip --disable-executable-stripping --disable-library-stripping --disable-library-profiling --disable-profiling --enable-static --enable-shared --disable-coverage --enable-library-for-ghci --datadir=/nix/store/p607d39a632lsp87wc21mckrcijb47ch-unix-lib-unix-2.7.2.2-data/share/ghc-9.2.5 --enable-split-sections
       > Configuring library for unix-2.7.2.2..
       > Setup: Encountered missing or private dependencies:
       > base >=4.5 && <4.11,
       > bytestring >=0.9.2 && <0.11,
       > time >=1.2 && <1.9
       >
       For full logs, run 'nix log /nix/store/xw1lbyk17ywd4yj2724ib5m7ljikwf4a-unix-lib-unix-2.7.2.2.drv'.

This is a bug I introduced in #1775. I adjusted the package builder to use the cabal file directly from the cabal plan (to support revisions) but I didn't consider that stackage snapshots (from stackage.nix) can only get the cabal file revisions from hackage.nix (since cabal is not involved in making the plan).