x/build: spurious cannot find package on plan9-arm

[bcmills]

go/src/runtime/cgocall.go:90:2: cannot find package "runtime/internal/sys" in:
	/boot/workdir/go/src/runtime/internal/sys
go tool dist: Failed: exit status: 'go 11148: 1'

greplogs -l -e '(?ms)\Aplan9.* cannot find package' --since=2022-01-01
2022-05-13T19:51:03-540f8c2/plan9-arm

Looks like probably a filesystem bug to me..?

(attn @golang/plan9)