cmd/dist: disable plugin test on linux-arm with GOARM=5

cmd/dist: disable plugin test on linux-arm with GOARM=5

Plugin support is patchy at the moment, so disable the test for
now until the test can be fixed. This way, we can get builders
for ARMv5 running for the rest of the code.

Updates #19674

Change-Id: I08aa211c08a85688656afe2ad2e680a2a6e5dfac
Reviewed-on: https://go-review.googlesource.com/39716
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>