Gracefully handle missing USER env var

Prompted by https://github.com/Homebrew/install/pull/213

Library/Homebrew/brew.sh

@@ -4,6 +4,11 @@ then
   export LC_ALL="en_US.UTF-8"
 fi
 
+# The USER env var is not guaranteed to be available, so fall back to id if not
+# provided. We destructively set it in the environment to make it available to
+# any sub-processes called.
+export USER=${USER:-`id -un`}
+
 # Where we store built products; a Cellar in HOMEBREW_PREFIX (often /usr/local
 # for bottles) unless there's already a Cellar in HOMEBREW_REPOSITORY.
 if [[ -d "$HOMEBREW_REPOSITORY/Cellar" ]]