x/build/env: host-plan9-386-gce (VM Image: plan9-386-v7)

[joedian]

[bcmills]

Note that the previous attempt at a plan9/386 builder on GCE didn't work reliably; see #29801.

(I suspect that this builder will need significant attention from @golang/plan9.)

[dmitshur]

I don't think anyone has plans to resume work on this, and there is a working plan9/386 LUCI builder tracked in #63599. Closing this in favor of that issue.