`internal error`s when removing caches

[shonfeder]

https://ocaml.ci.dev/github/ocurrent/ocaml-ci/commit/392877af1d5dc45ab0a4a8cf811e72afbed7f9e9/variant/alpine-3.19-4.14_opam-2.1/-/refs/pull/937/head

2024-05-27 17:05.14 ---> using "e0b7c871ec9de0848170f20b4e6e86ace8241db11324425b4b0e8149fc59e32a" from cache


/src: (copy (src .) (dst /src))
Uncaught exception: Failure("\"rm\" \"-rf\" \"/var/cache/obuilder/merged/065262520cb42e8446ff8309a4ccfb902086ca8723a4f87703bc60ddfac12224\" \"/var/cache/obuilder/work/065262520cb42e8446ff8309a4ccfb902086ca8723a4f87703bc60ddfac12224\" failed with exit status 1")
2024-05-27 17:05.15: Job failed: Failed: Internal error

https://ocaml.ci.dev/github/ocurrent/ocaml-ci/commit/392877af1d5dc45ab0a4a8cf811e72afbed7f9e9/variant/fedora-38-5.2_opam-2.1

/src: (copy (src .) (dst /src/))
Uncaught exception: Failure("\"rm\" \"-rf\" \"/var/cache/obuilder/merged/bdf8e9a9f488ea8dbc3aa99206515b021750bca54ffb2a4e2066bcc919035785\" \"/var/cache/obuilder/work/bdf8e9a9f488ea8dbc3aa99206515b021750bca54ffb2a4e2066bcc919035785\" failed with exit status 1")
2024-05-27 17:05.09: Job failed: Failed: Internal error