analytics: make Linux UUID generation more robust

Not every Linux system has a pre-installed `uuidgen` command (Debian
has a package `uuid-runtime` but doesn't install it by default). In
contrast, `/proc/sys/kernel/random/uuid` is always available on a modern
Linux system and produces type/version 4 UUIDs, just like `uuidgen`.

Closes #416.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/utils/analytics.sh

@@ -35,7 +35,12 @@ setup-analytics() {
   HOMEBREW_ANALYTICS_USER_UUID="$(git config --file="$git_config_file" --get homebrew.analyticsuuid)"
   if [[ -z "$HOMEBREW_ANALYTICS_USER_UUID" ]]
   then
-    HOMEBREW_ANALYTICS_USER_UUID="$(uuidgen)"
+    if [[ -n "$HOMEBREW_LINUX" ]]
+    then
+      HOMEBREW_ANALYTICS_USER_UUID="$(tr a-f A-F < /proc/sys/kernel/random/uuid)"
+    else
+      HOMEBREW_ANALYTICS_USER_UUID="$(uuidgen)"
+    fi
 
     if [[ -z "$HOMEBREW_ANALYTICS_USER_UUID" ]]
     then