discourage casual Big Sur installs

scripts/install-nix-from-closure.sh

@@ -27,6 +27,15 @@ if [ "$(uname -s)" = "Darwin" ]; then
     IFS='.' read macos_major macos_minor macos_patch << EOF
 $(sw_vers -productVersion)
 EOF
+    # TODO: this is a temporary speed-bump to keep people from naively installing Nix
+    # on macOS Big Sur (11.0+, 10.16+) until nixpkgs updates are ready for them.
+    # *Ideally* this is gone before next Nix release. If you're intentionally working on
+    # Nix + Big Sur, just comment out this block and be on your way :)
+    if [ "$macos_major" -gt 10 ] || { [ "$macos_major" -eq 10 ] && [ "$macos_minor" -gt 15 ]; }; then
+        echo "$0: nixpkgs isn't quite ready to support macOS $(sw_vers -productVersion) yet"
+        exit 1
+    fi
+
     if [ "$macos_major" -lt 10 ] || { [ "$macos_major" -eq 10 ] && [ "$macos_minor" -lt 12 ]; } || { [ "$macos_minor" -eq 12 ] && [ "$macos_patch" -lt 6 ]; }; then
         # patch may not be present; command substitution for simplicity
         echo "$0: macOS $(sw_vers -productVersion) is not supported, upgrade to 10.12.6 or higher"