Fix false "BUILD FAILED" message when installing Rubinius

Installing Rubinius would print "BUILD FAILED" at the `bundle` step but
the installation would proceed nevertheless and finish successfully.

It turns out that the `command ...` expression will trigger the ERR trap
on failure even when it's a part of `||` list or a condition inside an
`if` statement. This defies how the ERR trap is supposed to work, so the
workaround is to execute it in a subshell which doesn't inherit the ERR
trap of its parent.

bin/ruby-build

@@ -743,11 +743,13 @@ bundle() {
 }
 
 isolated_gem_dependency() {
-  local output="$(command $1 2>/dev/null || true)"
-  if [ -z "$output" ]; then
+  set +E
+  ( command $1 &>/dev/null ) || {
+    set -E
     shift 1
     isolated_gem_install "$@"
-  fi
+  }
+  set -E
 }
 
 isolated_gem_install() {