also strip '--mkfs-time 0'

Apparently appimagetool passes '--mkfs-time 0' to `mksquashfs`. It does this to make builds reproducible, but we set SOURCE_DATE_EPOCH for this purpose. If *both* are passed, then `mksquashfs`, exits with an error.

To be consistent, this commit just strips the `--mkfs-time 0` option

 * #78 (comment)
 * AppImage/AppImageKit#1202

build/linux/buildAppImage.sh

@@ -469,6 +469,7 @@ export SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH}"
 # the new version of mksquashfs no longer supports the '-mkfs-fixed-time'
 # argument, so we remove it
 args=\$(echo "\$@" | sed -e 's/-mkfs-fixed-time 0//')
+args=\$(echo "\$@" | sed -e 's/-mkfs-time 0//')
 $(which mksquashfs) \$args
 EOF