Revert "Prefix the generated XPI file with timestamp"

This reverts commit 97c698b6ae3709d58ec4876d4eebf99cf8bde77e.

Will not be needed since we are going to separate building langpack from
downloading it.
Use lock file to prevent concurrent build_xpi calls

Attempting to run build_xpi concurrently can fail with non-obvious
errors, so it is petter to limit it to one build at time.
Separate building and downloading langack actions

So that we can allow everyone to download the packs, but only few to
build them.