pack builder suggest should not panic when using registry mirror

[aemengo]

Summary

Running pack builder suggest with a registry mirror configured causes a golang panic errror

19:23 $ pack builder suggest

...

Using mirror <REDACTED-REGISTRY-MIRROR>/dockerhub-mirror/heroku/buildpacks:18 for heroku/buildpacks:18
Using mirror <REDACTED-REGISTRY-MIRROR>/dockerhub-mirror/paketobuildpacks/builder:base for paketobuildpacks/builder:base
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x185090c]

goroutine 18 [running]:
github.com/buildpacks/pack/internal/commands.getBuilderDescription(0x1a50ab5, 0x6, 0x1a65f26, 0x1c, 0x1a65896, 0x1c, 0x1bb0060, 0xc000382300, 0x0, 0x0)
	github.com/buildpacks/pack/internal/commands/suggest_builders.go:121 +0x9c
github.com/buildpacks/pack/internal/commands.WriteSuggestedBuilder.func2(0x1bb0060, 0xc000382300, 0xc00042a0c0, 0x6, 0x6, 0xc00041c190, 0x0, 0x1a50ab5, 0x6, 0x1a65f26, ...)
	github.com/buildpacks/pack/internal/commands/suggest_builders.go:102 +0x60
created by github.com/buildpacks/pack/internal/commands.WriteSuggestedBuilder
	github.com/buildpacks/pack/internal/commands/suggest_builders.go:101 +0x283

---

Reproduction

Steps

1. Configure pack with registry mirror
2. Run pack builder suggest

Current behavior

• The suggested builders do not show
• A panic error is thrown

Expected behavior

• The suggested builders should be shown
• A panic error should not be thrown

---

Environment

pack info

$ pack report
Pack:
  Version:  0.20.0+git-66a4f32.build-2668
  OS/Arch:  darwin/amd64

Default Lifecycle Version:  0.11.3

Supported Platform APIs:  0.3, 0.4, 0.5, 0.6

Config:
  default-builder-image = "[REDACTED]"
  experimental = true

  [registry-mirrors]
    "*" = "<REDACTED-REGISTRY-MIRROR>/dockerhub-mirror"