missing sticky bit in PEBBLE_PATH
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Issue body
### Check existing issues
- [x] I've verified that this bug isn't described by any existing issues.
### Bug description
May not be a bug, but definitely a security-related consideration that may need to be revisited, to ensure it was either done on purpose, and if so, to document why, or if not, to fix it.
The original thread comes from https://github.com/dotnet/dotnet-docker/issues/7054.
Once addressed, please provide feedback in that thread.
### Steps to reproduce
`rockcraft pack` any rock
### Environment
Should be the case on any rockcraft version and underlying environment, since the culprit is the implicit Pebble part.
### rockcraft.yaml
```yaml
```
### Log output
```shell
```
Evaluation history
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