Fix get_current_version

When some packages share the same base name the get_current_version may return multiple version number which is an error.

build.sh

@@ -179,7 +179,7 @@ get_current_version()
 		popd > /dev/null
 	fi
 
-	grep "$1" "$SRCDIR/current-repo/current" | cut -f 2
+	grep -P "^$1\t" "$SRCDIR/current-repo/current" | cut -f 2
 }
 
 get_aur_version()