Fix devx wrapper: set $out for stdenv/setup outside Nix builds

stdenv's setup.sh calls _assignFirst which requires $out to be set
for output variable assignment. Inside a Nix build the builder sets
$out automatically, but when running the devx wrapper directly
(containers, CI validation) $out is unset and setup.sh fails with:

  error: _assignFirst: could not find a non-empty variable whose
  name to assign to outputDev.

Fix in two places:
- mkEnvScript: wrapper sets $out to a temp dir when unset
- pr-validate.yml: set $out before invoking cached wrappers
  that don't yet include the mkEnvScript fix

Author: angerman

.github/workflows/pr-validate.yml

@@ -99,6 +99,10 @@ jobs:
         echo "Cabal: $(cabal --version 2>/dev/null | head -1 || echo N/A)"
         TESTEOF
 
+        # stdenv's setup.sh requires $out for output variable assignment.
+        # Inside a Nix build $out is set automatically; when running
+        # directly we must provide a dummy value.
+        export out=$(mktemp -d)
         "$DEVX_PATH" "$SMOKE_TEST"
         echo "::endgroup::"
 

flake.nix

@@ -349,6 +349,14 @@
                   # attributes needed to initialize the full dev environment.
                   ${envExports}
 
+                  # setup.sh requires $out for output variable assignment
+                  # (_assignFirst). Inside a Nix build $out is set by the
+                  # builder; when running directly (container, CLI) we
+                  # provide a temporary directory so setup.sh succeeds.
+                  if [ -z "''${out:-}" ]; then
+                    export out=$(mktemp -d)
+                  fi
+
                   # Source stdenv's setup.sh to initialize the development
                   # environment. This runs all setup hooks (cc-wrapper,
                   # pkg-config-wrapper, etc.) and populates NIX_CFLAGS_COMPILE,