Cabal fails to override <pkg>_datadir

.../alex-3.5.3.0> strace -f -e execve -v -s4096 -o trace -E alex_datadir=foo ./Setup test -v
Reading installed packages...
Running: /usr/bin/ghc-pkg dump --global -v0
Running: /usr/bin/ghc-pkg dump '--package-db=dist/package.conf.inplace' -v0
Running: /usr/bin/ghc --print-libdir -fobject-determinism -fhide-source-paths
Running 2 test suites...
Test suite tests-debug: RUNNING...
Running: dist/build/tests-debug/tests-debug
make: Entering directory '/home/abuild/rpmbuild/BUILD/alex-3.5.3.0-build/alex-3.5.3.0/tests'
which: no alex in (/usr/local/bin:/usr/bin:/bin)
rm -f *.n.hs *.g.hs *.d.hs *.info *.hi *.o *.bin *.exe
../dist/build/alex/alex --debug basic_typeclass.x -o basic_typeclass.d.hs
alex: Uncaught exception ghc-internal:GHC.Internal.IO.Exception.IOException:

foo/AlexWrappers.hs: openFile: does not exist (No such file or directory)

HasCallStack backtrace:
ioError, called at libraries/ghc-internal/src/GHC/Internal/Foreign/C/Error.hs:291:5 in ghc-internal:GHC.Internal.Foreign.C.Error

make: *** [Makefile:109: basic_typeclass.d.hs] Error 1
...
.../alex-3.5.3.0> sed -rn 's/^.*execve\("([^"]*)".*(alex_datadir=[^"]*).*(alex_datadir=[^"]*).*$/\1 \2 \3/p' trace
dist/build/tests-debug/tests-debug alex_datadir=/home/abuild/rpmbuild/BUILD/alex-3.5.3.0-build/alex-3.5.3.0/data/ alex_datadir=foo
/bin/sh alex_datadir=/home/abuild/rpmbuild/BUILD/alex-3.5.3.0-build/alex-3.5.3.0/data/ alex_datadir=foo
dist/build/tests/tests alex_datadir=/home/abuild/rpmbuild/BUILD/alex-3.5.3.0-build/alex-3.5.3.0/data/ alex_datadir=foo
/bin/sh alex_datadir=/home/abuild/rpmbuild/BUILD/alex-3.5.3.0-build/alex-3.5.3.0/data/ alex_datadir=foo