ci/check-warnings: ignore warnings that we're using the mirror

SourceForge in particular doesn't like multiple connections from the
same IP, but when a source mirror is being populated for the first time
this will happen. Our fetch falls back to the Yocto mirror so it doesn't
cause a failure, but there is a warning logged which our CI then fails
because of.

As we don't care about these warnings, filter them out of the log before
checking if there were any errors.

Change-Id: I36c97c5d9923f1c4d14c4588f3780211cccb57b2
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>

ci/check-warnings

@@ -7,7 +7,8 @@ set -e -u
 
 LOGFILE=$1
 
-if test -s $LOGFILE; then
+LINES=$(grep --invert-match "attempting MIRRORS if available" $LOGFILE | wc -l)
+if test "$LINES" -ne 0; then
     echo ==============================
     echo The build had warnings/errors:
     echo ==============================