x/build: add LUCI plan9-amd64 builder

[0intro]

• Hostname: plan9-amd64
• Certificate Request: plan9-amd64.csr.txt

[cagedmantis]

@golang/release

[gopherbot]

Change https://go.dev/cl/536196 mentions this issue: main.star: add plan9-{386, amd64, arm} builders

[cagedmantis]

Here is the certificate: plan9-amd64-1698251554.cert.txt

[cagedmantis]

@0intro Have you had a chance to test out the bot with the certificate?

[0intro]

@cagedmantis Yes. But since the bot can't run on Plan 9 directly (there is no support for Python on Plan 9), we would like to get some help to make changes to the bot, so it could be able to run on a Linux machine and send commands to a remote Plan 9 instance through SSH.

See #62025 and #63599.