Fix compression for non-GNU tar (#782)

This fixes a problem reported on MacOS, where the "standard" tar is BSD tar.
That one interprets the `-I` argument differently.

alibuild_helpers/build_template.sh

@@ -292,9 +292,9 @@ if [ "$CAN_DELETE" = 1 ]; then
   rm -f "$WORK_DIR/TARS/$HASH_PATH/$PACKAGE_WITH_REV"
 elif [ -z "$CACHED_TARBALL" ]; then
   # We don't have an existing tarball, and we want to keep the one we create now.
-  # Avoid having broken left overs if the tar fails
-  tar -cf "$WORK_DIR/TARS/$HASH_PATH/$PACKAGE_WITH_REV.processing" \
-      -C "$WORK_DIR/INSTALLROOT/$PKGHASH" -I "$MY_GZIP" .
+  tar -cC "$WORK_DIR/INSTALLROOT/$PKGHASH" . |
+    # Avoid having broken left overs if the tar fails.
+    $MY_GZIP -c > "$WORK_DIR/TARS/$HASH_PATH/$PACKAGE_WITH_REV.processing"
   mv "$WORK_DIR/TARS/$HASH_PATH/$PACKAGE_WITH_REV.processing" \
      "$WORK_DIR/TARS/$HASH_PATH/$PACKAGE_WITH_REV"
   ln -nfs "../../$HASH_PATH/$PACKAGE_WITH_REV" \