style test relies on adjacent dmd repo being present

[MartinNowak]

Fails with fresh on-demand agents when make -f posix.mak style tries to include ../dmd/osmodel.mak.
https://buildkite.com/dlang/phobos/builds/552#100bbbc3-6b0d-44af-861d-99a0791563dc
Just didn't happen on the long-running buildkite agents because they still had a previous dlang/dmd repo leftover.
The whole folder setup is quite messy atm. and we should consider to rewrite it, using a custom noop checkout hook and doing the checkouts manually into an empty working directory.
Checkout hooks can be installed on the agents under /etc/buildkite-agent/hooks/checkout.
This would also avoid checkouts for every project being tested.

[MartinNowak]

@wilzbach

[wilzbach]

This was fixed with #340. However, now it fails because DMD is segfaulting while building Dscanner.
This has been fixed at Dscanner and there's a PR to update Dscanner at Phobos: dlang/phobos#6783

Yes this is coming. Maybe just a few filtering rules to adjust.

[wilzbach]

The whole folder setup is quite messy atm. and we should consider to rewrite it, using a custom noop checkout hook and doing the checkouts manually into an empty working directory.

Moved this to #358

[wilzbach]

Closing as I moved the main issue to #358 and the style test have been fixed.