x/build/env: host-openbsd-386-* (VM Image: openbsd-386-*)

[joedian]

This issue tracks porting the openbsd-386-* (OpenBSD with GOARCH=386) GCE host image to LUCI.

Issue #63568 tracks updating to OpenBSD 7.3/7.4 (which might supersede this 7.2 version).
Issue #69823 tracks updating to OpenBSD 7.6.

[bcmills]

(CC @golang/openbsd)

[4a6f656c]

The work for this should already be done as the build scripts for openbsd/386 and openbsd/amd64 are shared. Given that the openbsd/amd64 builder is already running on LUCI, it should just be a matter of building, pushing and testing the i386 image.