Fix env-test for cross-compilation variants (static, JS)

Cross-compilation GHC packages only have target-prefixed binaries
(e.g. x86_64-unknown-linux-musl-ghc, javascript-unknown-ghcjs-ghc)
with no unprefixed bin/ghc symlink. Extract the correct GHC command
from NIX_CABAL_FLAGS --with-ghc=... which is set by the env script
for all cross-compilation shells.

Author: angerman

flake.nix

@@ -414,8 +414,20 @@
                 # Source the environment script to set up PATH and env vars
                 source ${envScript}
 
+                # For cross-compilation shells (static/musl, JS), the GHC binary
+                # has a target prefix (e.g. x86_64-unknown-linux-musl-ghc).
+                # Extract the correct name from NIX_CABAL_FLAGS if set.
+                GHC_CMD="ghc"
+                if [[ -n "''${NIX_CABAL_FLAGS:-}" ]]; then
+                  for flag in $NIX_CABAL_FLAGS; do
+                    case "$flag" in
+                      --with-ghc=*) GHC_CMD="''${flag#--with-ghc=}" ;;
+                    esac
+                  done
+                fi
+
                 echo "=== Testing ${name} ==="
-                echo -n "GHC: "; ghc --version
+                echo -n "GHC: "; $GHC_CMD --version
                 echo -n "Cabal: "; cabal --version | head -1
                 echo -n "pkg-config packages: "; pkg-config --list-all | wc -l
                 ${lib.optionalString hasHLS ''