general.aliases: Run which gshuf in if condition

If .bashrc is sourced while `set -e` is set, and `gshuf` is not installed,
then running `which gshuf` will cause the shell to error and exit.
Running `which gshuf` during an if condition will capture the error.

aliases/available/general.aliases.bash

@@ -27,8 +27,7 @@ then
   export GREP_COLOR='1;33'
 fi
 
-which gshuf &> /dev/null
-if [ $? -eq 0 ]
+if which gshuf &> /dev/null
 then
   alias shuf=gshuf
 fi