getaddrinfo: fix CGO_ENABLED=0 and record resolver type

[bassosimone]

After #764, the build for
CGO_ENABLED=0 has been broken for miniooni:

https://github.com/ooni/probe-cli/runs/6636995859?check_suite_focus=true

Likewise, it's not possible to run tests with CGO_ENABLED=0.

To make tests work with CGO_ENABLED=0, I needed to sacrifice some
unit tests run for the CGO case. It is not fully clear to me what was happening
here, but basically getaddrinfo_cgo_test.go was compiled with CGO
being disabled, even though the ``//go:build cgo` flag was specified.

Additionally, @hellais previously raised a valid point in the review
of #698:

Another issue we should consider is that, if I understand how
this works correctly, depending on whether or not we have built
with CGO_ENABLED=0 on or not, we are going to be measuring
things in a different way (using our cgo inspired getaddrinfo
implementation or using netgo). This might present issues when
analyzing or interpreting the data.

Do we perhaps want to add some field to the output data format that
gives us an indication of which DNS resolution code was used to
generate the the metric?

This comment is relevant to the current commit because
#698 is the previous
iteration of #764.

So, while fixing the build and test issues, let us also distinguish
between the CGO_ENABLED=1 and CGO_ENABLED=0 cases.

Before this commit, OONI used "system" to indicate the case where
we were using net.DefaultResolver. This behavior dates back to the
Measurement Kit days. While it is true that ooni/probe-engine and
ooni/probe-cli could have been using netgo in the past when we
said "system" as the resolver, it also seems reasonable to continue
to use "system" top indicate getaddrinfo.

So, the choice here is basically to use "netgo" from now on to
indicate the cases in which we were built with CGO_ENABLED=0.

This change will need to be documented into ooni/spec along with
the introduction of the android_dns_cache_no_data error.

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: android: properly handle NXDOMAIN errors probe#2029
• if you changed anything related how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request: formats: document dns errors and resolver types spec#242