The Linux tests under the Cirrus CI are currently brittle due to
environmental issues. They're also redundant, since Linux testing is
much better covered by the GitLab CI tests.
So this patch removes them, which removes the false positives and speeds
up the Cirrus CI runs.
Cirrus CI caches the module directory inside $GOPATH so it can be shared
between test runs, to speed them up.
However, it is currently created as root, which causes new entries to
fail since they run under the "testing" user.
This patch fixes that problem by making the "testing" user own the
entire $GOPATH, which includes the module directory.
tls.Config.BuildNameToCertificate was deprecated in Go 1.14, and is no
longer necessary.
However, we support down to 1.11, so we will keep it for now.
This patch adds a TODO to remove it in the future once the minimum
supported version is 1.14; and adjust the CI linter accordingly.
