More fixes for golint.sh

[Groxx]

I have no idea why invalid sh syntax isn't a script failure, but the checkmark is still green:

./scripts/buildkite/golint.sh: 12: [[: not found
+ git status --porcelain
fatal: detected dubious ownership in repository at '/cadence'
To add an exception for this directory, call:
	git config --global --add safe.directory /cadence
+ [ -n  ]

and apparently that documented environment variable doesn't actually exist.
BUILDKITE=true exists though and it seems reasonable.

and I forgot that >&1 only captures stdout (and redirects it to stdout), so stderr was still being missed.

this is all so very crazy.

but after throwing enough stuff at the wall and testing with a non-clean git status, this appears to work and makes sense.
this should also catch "dir changed and safe-directory no longer works", unlike before, so we can notice and fix it more quickly.