Make sure ~/.bitcoin doesn't exist before build

[meshcollider]

We've been getting some random travis failures since #11260 was merged, because the ~/.bitcoin directory exists after tests are run. Not sure exactly what's causing it, but this PR ensures the directory doesn't exist before running the build and tests, to see if this fixes the issue.

Edit: travis has been run on this merge twice, and all tests passed both times, so either this fixes the issue or it just got lucky

[sipa]

utACK b73628d. It survived 2 full Travis runs, and fixed the same issue occurring in #11167.

[laanwj]

If this is what I think it is, that PRs that were built by travis and cached before #11210 was merged have a ~/.bitcoin in their cache, it can also be solved by wiping the Travis caches.

Edit: had to wipe the entire travis cache to test this - the Travis interface made it impossible to select individual PRs beyond 11111 or so. So I wiped all the cache and started a new build for the branches. Builds will be slow until master finishes rebuilding.

[meshcollider]

@laanwj does Travis cache data only from builds which pass all tests?

[laanwj]

@meshcollider Yes

[meshcollider]

@laanwj In that case this should never be an issue again right, because there should be no way for the directory to ever be cached, so we can close this

[laanwj]

I hope so! I'm not sure this is actually the issue, but it seemed likely. I do not understand where a .bitcoin directory would come from otherwise. We still might merge this though for symmetry, I don't know.

[laanwj]

I don't think this was solved, I started #11289 a few times and it's still failing. Reopening.

[maflcko]

Going to merge this, so we gain some air to breathe. But I can't see how this dir was created at that point.

[meshcollider]

#11289 is passing now 👍

[laanwj]

Great! I think this is good for a temporary workaround.

There's no chance that travis.yml will be executed on user's account, otherwise this would be terribly dangerous.