hadrian: Fix build-stack-nix

As noted by #19589, `stack` is not stateful and therefore must be passed
`--nix` on every invocation. Do so.

Fixes #19589.

hadrian/build-stack

@@ -7,9 +7,9 @@ STACK="${STACK:-stack}"
 
 # Make sure Hadrian is up-to-date
 cd hadrian
-$STACK build --no-library-profiling ${HADRIAN_NIX:+--nix}
+$STACK build --no-library-profiling "${HADRIAN_NIX:+--nix}"
 
 # Run Hadrian in the top-level GHC directory
-$STACK exec hadrian -- \
+$STACK exec hadrian "${HADRIAN_NIX:+--nix}" -- \
     --directory ".."  \
     "$@"