switch to more compatible bc expr

ci-coverage.sh

@@ -4,7 +4,7 @@ THRESHOLD=75 # percentage
 
 go test -coverprofile c.out ./...
 cov=$(go tool cover -func c.out  | grep '^total:' | awk '{ print $3+0 }')
-if [[ $(bc --expression="$cov < $THRESHOLD") ]]; then
+if [[ $(echo "$cov < $THRESHOLD" | bc) ]]; then
     echo "threshold not met - code must be at or above $THRESHOLD% coverage"
     exit 1
 fi