cmd/dist: leave cgo enabled if external linking is required
Certain ios and android configurations do not yet support internal
linking.
On ios, attempting to build without cgo causes tests to fail on
essentially every run (#57961).
On android, it produces a lot of warning spam from the linker,
obscuring real problems.
Since external linking makes the result of `go install` depend on the
installed C toolchain either way, the reproducibility benefit of
disabling cgo seems minimal on these platforms anyway.