Adds flock for concurrent access

[xtuc]

Download does the following steps:

1. defines a destination (return if already exists)
2. download and extract in a temporary directory
3. rename the temporary directory to the final destionation

Now imagine a concurrent install is running, if after the step 1. another
installation succedeed the rename (step 3.) will fail
(however, only under Linux because of different syscalls).

This change allows to acquire a lock before step 1 and release it
after step 3. Concurrent access will wait.

Fixes #2

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