Every time before sending a PR I like to run ./scripts/build-test-lint.sh to make sure the CI won't complain about anything.
The problem is that this script attempts to install golangci-lint, which causes modifications to go.mod/go.sum. This PR backs up and restores those files before and after linting.
Ideally instead of this hacky backing up/restoring we'd use go gets -mod=readonly option, but that still modifies go.sum. This will be fixed in go 1.13 apparently. golang/go#30667
We should no longer be building anything into vendor/bin in the scripts,
so adding it to the path can lead to all sorts of confusion if old
binaries are there.
The motivation for this is to make it easier to see whether a travis failure is due to linting, unit tests or integration test failures, without having to look in the logs.
It also means that each job is independent, so if e.g. the linting fails then the unit tests will still be run.
