Fix cronjobs not setting USER variable

I start ./loop_gdb.sh with a cronjob set in `crontab -e`
it runs as a non root linux user.
But cron does not set the environment variable USER
I could in the `crontab -e` at the top manually do `USER=chiller`
but there is a risk I forget this.

I tested `whoami` instead of `$USER` and it actually seemed to be set.
Also `id` showed the name but I found no flag to only print the name.

So from my quick test it seems like `whoami` is more portable than
then `$USER` variable.

Fixes the following error I got:

	rm: remove write-protected regular file '/tmp/_ddpp_build_lock.txt.lock'?

You can see the missing username in front of the leading _
that is because the $USER expanded to the empty string because the
./loop_gdb.sh was started from cron.
Then this file was shared between multiple users and the `mv`
command became interactive. Because the underlying `rm` command
noticed a permissions change.
Could also do a `rm -f` here but the actual issue is that we
shared the lock file for multiple users.

Author: ChillerDragon

lib/include/build_lock.sh

@@ -11,7 +11,7 @@
 #
 # unlock_build
 
-BUILD_LOCKFILE=/tmp/"$USER"_ddpp_build_lock.txt
+BUILD_LOCKFILE=/tmp/"$(whoami)"_ddpp_build_lock.txt
 
 ###################
 # internal helpers