plugin: fix build constraint to disable test on linux/arm64
plugin: fix build constraint to disable test on linux/arm64

CL 151478 was suppose to fix the build failure on linux/arm64 but the
build constraint didn't exclude linux/arm64 properly.

Fixes #28982

Change-Id: Ia80265b0adba0384cd28bc2deb1726418664975a
Reviewed-on: https://go-review.googlesource.com/c/151303
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,golang/go,golang/go,josharian/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,nawawi/go,nawawi/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,josharian/go,nawawi/go,CAFxX/go,golang/go,akutz/go,golang/go,golang/go,nawawi/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go