add _opam to ignored directories for file check

1 job for master in 15 minutes and 58 seconds (queued for 16 seconds)
Name Stage Failure
failed Build

The former state can be restored with:
opam switch import "/builds/iris/refinedc/_opam/ocaml-base-compiler.4.07.1/.opam-switch/backup/state-20220126030618.export"
Or you can retry to install your package selection with:
opam install --restore
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1