x/build: update plan9-arm builder to a current buildlet binary

[bcmills]

What did you do?

~/go/src$ gomote run -dir=/boot/workdir/go/src bcmills-plan9-arm-0 ./make.rc

What did you expect to see?

Successful make.rc run.

What did you see instead?

~/go/src$ gomote run -dir=/boot/workdir/go/src bcmills-plan9-arm-0 ./make.rc
Error running run: Error trying to execute ./make.rc: unable to execute command: buildlet: HTTP status 400 Bad Request: bogus 'dir' parameter

The “bogus 'dir parameter'” error message was removed in https://go.dev/cl/367896, so this seems to imply that the plan9-arm builder is not using an up-to-date cmd/buildlet binary. (I had through that cmd/buildlet automatically updated itself, but I guess not?)

(attn @0intro @millerresearch)

[0intro]

I've just rebuilt the buildlet binaries:

• http://9legacy.org/download/go/buildlet/386/buildlet
• http://9legacy.org/download/go/buildlet/amd64/buildlet
• http://9legacy.org/download/go/buildlet/arm/buildlet

I've updated the buildlet binary on the plan9-386 and plan9-amd64 builders.

@millerresearch Could you please update the buildlet binaries on the plan9-arm builders?

Thanks.

[millerresearch]

Done.