Document 'GOKRAZY_PARENT_DIR' environment variable about gokrazy

[seanhagen]

Also, just found out that gok overwrite apparently doesn't fully use GOKRAZY_PARENT_DIR:

$ gok -i hello overwrite --full /dev/disk/by-id/wwn-0x5002538043584d30                                                                                                                                                                             Sun 24 Sep 2023 12:08:29 AM
gokrazy gok g231dea on GOARCH=amd64 GOOS=linux

Build target: CGO_ENABLED=0 GOARCH=arm64 GOOS=linux
Build timestamp: 2023-09-23T17:08:52-07:00
Loading system CA certificates from /etc/ssl/certs/ca-certificates.crt
Building 4 Go packages:

  github.com/gokrazy/fbstatus

  github.com/gokrazy/hello

  github.com/gokrazy/serial-busybox

  github.com/gokrazy/breakglass
    will be started with command-line flags
      from /home/sean/Code/GoKrazy/hello/config.json
      last modified: 2023-09-23T16:25:49-07:00 (43m4s ago)

[done] in 5.44s                        

Including extra files for Go packages:

  github.com/gokrazy/breakglass
    will include extra files in the root file system
      from /home/sean/Code/GoKrazy/hello/breakglass.authorized_keys
      last modified: 2023-09-23T16:25:49-07:00 (43m11s ago)

Including loadable kernel modules from:
/home/sean/Code/Go/pkg/mod/github.com/gokrazy/[email protected]/lib/modules

Feature summary:
  use GPT: true
  use PARTUUID: true
  use GPT PARTUUID: true
2023/09/23 17:09:01 partitioning /dev/disk/by-id/wwn-0x5002538043584d30 (GPT + Hybrid MBR)
2023/09/23 17:09:01 Using sudo to gain permission to format /dev/disk/by-id/wwn-0x5002538043584d30
2023/09/23 17:09:01 If you prefer, cancel and use: sudo setfacl -m u:${USER}:rw /dev/disk/by-id/wwn-0x5002538043584d30
2023/09/23 17:09:01 open /home/sean/gokrazy/hello/config.json: no such file or directory
2023/09/23 17:09:01 exit status 1