Regression: crash when reviewing an already merged PR

[GaetanLepage]

This regression was added in #426.
If the PR is merged, pull/<PR>/merge will be gone, and won't be able to run nixpkgs-review against a merged PR after this.

-> Attempting to fetch eval results from GitHub actions
-> Successfully fetched rebuilds: no local evaluation needed
$ git -c fetch.prune=false fetch --no-tags --force https://github.com/NixOS/nixpkgs master:refs/nixpkgs-review/0 pull/366544/merge:refs/nixpkgs-review/1
fatal: couldn't find remote ref pull/366544/merge
https://github.com/NixOS/nixpkgs/pull/366544 failed to build: Failed to fetch ('master', 'pull/366544/merge') from https://github.com/NixOS/nixpkgs. git fetch failed with exit code 128
$ git worktree remove -f /home/gaetan/.cache/nixpkgs-review/pr-366544/nixpkgs
fatal: '/home/gaetan/.cache/nixpkgs-review/pr-366544/nixpkgs' is not a working tree
Failed to remove worktree at /home/gaetan/.cache/nixpkgs-review/pr-366544/nixpkgs. Please remove it manually. Git failed with: 128