x/build: add LUCI plan9-arm builder

[0intro]

• Hostname: plan9-arm
• Certificate Request: plan9-arm.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-arm-1698265912.cert.txt

[gopherbot]

Change https://go.dev/cl/558517 mentions this issue: main.star: fix cipd_platform value for GOHOSTARCH=arm