brew.sh: avoid unnecessary 'cat'

Bash can read directly from a file in its command substitution.

Library/brew.sh

@@ -202,7 +202,7 @@ then
   HOMEBREW_ANALYTICS_USER_UUID_FILE="$HOME/.homebrew_analytics_user_uuid"
   if [[ -r "$HOMEBREW_ANALYTICS_USER_UUID_FILE" ]]
   then
-    HOMEBREW_ANALYTICS_USER_UUID="$(cat "$HOMEBREW_ANALYTICS_USER_UUID_FILE")"
+    HOMEBREW_ANALYTICS_USER_UUID="$(<"$HOMEBREW_ANALYTICS_USER_UUID_FILE")"
   else
     HOMEBREW_ANALYTICS_USER_UUID="$(uuidgen)"
     echo "$HOMEBREW_ANALYTICS_USER_UUID" > "$HOMEBREW_ANALYTICS_USER_UUID_FILE"