Avoid redundant work while hackily_snapshot()ing.

[stuhood]

Problem

In runs with high degrees of concurrency, we end up seeing many simultaneous instances of tools being fetched and snapshotted.

Solution

Enforce a single fetch+snapshot attempt per tool. As mentioned in the comment, this is a temporary solution until #7790 migrates this into the engine itself.

[cosmicexplorer]

Not super familiar with threading but this seems ideal! I would love to add a test which exercises the possibility of a multithreaded download (what this PR fixes), especially since that test would seem to still work upon conversion to v2 -- is there an easy way to do that by just kicking off two threads downloading a tool at once? If a test for that isn't easy or is likely to be flaky then I would love a TODO about that at least so we clearly know what things aren't being tested yet.

[cosmicexplorer]

Restarted a failing contrib shard!

[stuhood]

I would love to add a test which exercises the possibility of a multithreaded download (what this PR fixes), especially since that test would seem to still work upon conversion to v2 -- is there an easy way to do that by just kicking off two threads downloading a tool at once?

That would end up being a test of the rust Graph itself, rather than of downloading things. So while I agree that the test might be useful right now, it would not be in that future.