eclipses test instructions: don't rely on Nix experimental features

Prefer `nix-build`, which works out of the box, unlike `nix build`,
which fails until you enable experimental features.

pkgs/applications/editors/eclipse/default.nix

···
 # use ./update.sh to help with updating for each quarterly release
 #
 # then, to test:
-# for e in cpp dsl embedcpp modeling platform sdk java jee committers rcp; do for s in pkgs pkgsCross.aarch64-multiplatform; do echo; echo $s $e; nix build -f default.nix ${s}.eclipses.eclipse-${e} -o eclipse-${s}-${e}; done; done
+# for e in cpp dsl embedcpp modeling platform sdk java jee committers rcp; do for s in pkgs pkgsCross.aarch64-multiplatform; do echo; echo $s $e; nix-build -A ${s}.eclipses.eclipse-${e} -o eclipse-${s}-${e}; done; done
 
 let
   platform_major = "4";