feat!: Update output path writing

- Allow pull requests to create output paths

Previously, it was not possible to get unmerged pull request outputs. I
would like to access these (e.g. for hosting a preview version of a
website in a similar way to https://docs.netlify.com/site-deploys/deploy-previews/
or https://vercel.com/docs/deployments/preview-deployments)

These are now surfaced under `{owner}/{repo}/pulls/{pr number}`

- Fix repository attribute name conflicts

Previously there was no difference between samely-named attributes in
different repositories.

This has been changed so outputs are under `{owner}/{repo}/{branch}`

- Allow full attribute names, still stripping path traversal

Previously, presumably to prevent path traversal, if your attribute name
contained slashes buildbot-nix would only take the last segment as an
output.

This has been replaced by interpreting slashes as subdirectories and
refusing any segments which don't descend into a subdirectory (i.e. are
attempting path traversal)

- Still create outputs on skipped builds

Previously when something was skipped, for example a build that was
completed in a pull request, the output wouldn't be updated. This made
the outputs directory quite unreliable.

Outputs will now always be updated, no matter whether a build was
actually executed.

BREAKING-CHANGE: This stops old output locations being outputted. If you rely on these locations, you will need to update whatever relies on them.

buildbot_nix/__init__.py

@@ -339,11 +339,6 @@ class UpdateBuildOutput(steps.BuildStep):
         if not pr and props.getProperty("branch") != self.project.default_branch:
             return util.SKIPPED
 
-        out_path = props.getProperty("out_path")
-
-        if not out_path: # if, e.g., the build fails and doesn't produce an output
-            return util.SKIPPED
-
         owner = Path(props.getProperty("owner"))
         repo = Path(props.getProperty("repository_name"))
 
@@ -360,11 +355,11 @@ class UpdateBuildOutput(steps.BuildStep):
 
         file.parent.mkdir(parents=True, exist_ok=True)
 
+        out_path = props.getProperty("out_path")
         file.write_text(out_path)
 
         return util.SUCCESS
 
-
 # GitHub somtimes fires the PR webhook before it has computed the merge commit
 # This is a workaround to fetch the merge commit and checkout the PR branch in CI
 class GitLocalPrMerge(steps.Git):