don't fail in mkdir if path gets created while processing it

[Flamefire]

This may happen if a parallel build creates e.g. the sources-directory

Reported in Slack

I'm now seeing this error when I try to run a CI job (which runs EB to test installing some software)

HASH=$(git rev-parse HEAD) eb ci-plans/files/ligka_ci_actor.eb -f ${EB_OPTS} ${EB_HTTP_OPTS}
ERROR: Build of /home/ITER/haywart/src/ligka/ci-plans/files/ligka_ci_actor.eb failed (err: "build failed (first 300 chars): Failed to create directory /home/ITER/haywart/src/ligka/sources/generic/eb_v4.8.0/ConfigureMake: [Errno 17] File exists: '/home/ITER/haywart/src/ligka/sources/generic/eb_v4.8.0/ConfigureMake'")

Note that our mkdir is documented referring to mkdir -p which also doesn't fail if the directory exists.

[ocaisa]

LGTM
I don't see how we can (with reasonable effort) add a test for this, so if this fixes the issue you saw I'm happy to include it.

[Flamefire]

Ouch, I missed that FileExistsError is a Python 3 thing and doesn't exist in Python2