fix: use throttle: 1 for localhost unarchive

[travisdowns]

During the common install tasks, we download and unpack the binary to install on localhost and eventually upload it to each host (among many other things). Currently the "unpack" step, which extracts the gzipped tar archive, is perform N times if there are N hosts in the inventory, but the target directory (something like
/tmp/node_exporter-linux-arm64/1.8.2) is the same for every host with the same architecture.

This means that all the unarchive tasks are extracting in parallel in an unsynchronized manner to the same directory. It a miracle that this works, but at least usually it like it does, but sometimes tar will complain about a missing file, failing the install. I've confirmed locally that this is due to the race described above.

To fix this, we use throttle: 1 on the unpack task. This means that the first task (for each architecture) will do the unpack and the other tasks are effectively no-ops, they are reported as "ok" rather than changed in the output.

Fixes #457.

[travisdowns]

I tested this locally with a file that is more likely to trigger the race: a tar of 10,000 small files all at the top level of the tar: this triggers the error 100% of the time w/o the fix and passes 1,000 iterations locally with no failures.

I also verified that deploying prom to a 4 host inventory works, 100 iterations without failure. This inventory has 2 hosts each for 2 different architectures (amd64 and arm64).