ci: set CGO_ENABLED: 0 for arm/arm64 builds

[mikemorris]

Refs #7970 for context on the ARM crash in versions of Go prior to 1.14 that required enabling CGO.

As an open question, is the suggested approach in #6519 (comment) sufficient for running on ARMv7 hardware @jwalzer @mrmstn? It seems that approach (aside from the reported bug) has been working for some users according to #10545.

TODO: This change only affects our test suite, and will require a followup to our build pipelines if it looks good to merge.a

[alvin-huang]

With golang/go#34030 in Go 1.14, we should be able to compile arm/arm64 normally without having to download the gcc libraries IIRC. We just never switched it.

I don't know if we just need a linux_arm (default ARMv6) and linux_arm64 (aarch64/ARMv8) or if we need to support GOARM=5|6|7 as well.

[mikemorris]

This is obsolete as of #11417 merging, although we should probably remove the entire build-arm and other binary build sections from CircleCI config as a followup, preferring to instead validate binary builds using the new GitHub actions workflows.