Dangling symlinks cause `spread` to panic

[mr-cal]

Dangling symlinks in a spread test suite directory cause spread to panic with no useful error output. I can only reproduce this on self-hosted runners with the spread-installed tag.

Error:

Run spread google:ubuntu-24.04-64
  spread google:ubuntu-24.04-64
  shell: /usr/bin/bash -e {0}
  env:
    SPREAD_GOOGLE_KEY:
    UA_TOKEN: ***
    RUN_UA_TESTS: true
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x74bb25]
Error: Process completed with exit code 2.

I accidentally had a dangling symlink when removing a shared test like this:

tests/spread/
├── _common/
│   └── test-1/  ← deleted
├── core22/
│   └── test-1 -> ../_common/test-1  ← deleted
├── core24/
│   └── test-1 -> ../_common/test-1  ← deleted
└── core26/
    └── test-1 -> ../_common/test-1  ← dangling (forgotten, causes the panic)

See canonical/snapcraft#6198 and https://github.com/canonical/snapcraft/actions/runs/24582803735/job/71885536820?pr=6198 for a reproducer.