system: Default .bashrc now sources /etc/bashrc.

This is a followup to commit 1382bde93c,
which disabled automatic sourcing of /etc/bashrc by interactive shell.
The noticeable effect of the change would be that Bash completion would
only work in login shells.

* gnu/system/shadow.scm (default-skeletons)["bashrc"]: Source
/etc/bashrc.

gnu/system/shadow.scm

@@ -168,6 +168,9 @@ then
     return
 fi
 
+# Source the system-wide file.
+source /etc/bashrc
+
 # Adjust the prompt depending on whether we're in 'guix environment'.
 if [ -n \"$GUIX_ENVIRONMENT\" ]
 then