ci(test-virgin-user): preserve NIX_CONFIG across sudo to avoid GitHub API rate limits

https://chatgpt.com/share/6945565e-f1b0-800f-86d5-8d0083fe3390

.github/workflows/test-virgin-user.yml

@@ -49,11 +49,13 @@ jobs:
               chown -R dev:dev /nix
               chmod 0755 /nix
               chmod 1777 /nix/store
-
+              sudo -H -u dev env \
-              sudo -H -u dev env HOME=/home/dev PKGMGR_DISABLE_NIX_FLAKE_INSTALLER=1 bash -lc "
+                HOME=/home/dev \
+                NIX_CONFIG="$NIX_CONFIG" \
+                PKGMGR_DISABLE_NIX_FLAKE_INSTALLER=1 \
+                bash -lc "
                   set -euo pipefail
                   cd /opt/src/pkgmgr
-
                   make setup-venv
                   . \"\$HOME/.venvs/pkgmgr/bin/activate\"