Allow removing files that changed

[RalfJung]

If your issue concerns a package not building, please report to
https://github.com/ocaml/opam-repository/issues or to the package maintainer
unless you are confident it is an issue in the opam tool itself.

We just had CI fail with this error:

[WARNING] While removing coq-stdlib.dev: not removing files that changed since:
            - lib/coq/user-contrib/Ltac2/String.vo
            - lib/coq/user-contrib/Ltac2/Std.vo
            - lib/coq/user-contrib/Ltac2/Std.v
            - lib/coq/user-contrib/Ltac2/Printf.vo
            - lib/coq/user-contrib/Ltac2/Pattern.vo
            - lib/coq/user-contrib/Ltac2/Option.vo
            - lib/coq/user-contrib/Ltac2/Notations.vo
            - lib/coq/user-contrib/Ltac2/Notations.v
            - lib/coq/user-contrib/Ltac2/Message.vo
            - lib/coq/user-contrib/Ltac2/Ltac2.vo
            - lib/coq/user-contrib/Ltac2/Ltac1.vo

(and it goes on like this for a while)

These files were never touched outside of opam, so I am not sure why opam thinks that they were changed -- but even if they were changed, I am fine with opam removing them anyway. This is on CI, so I cannot just interactively delete them by hand. Deleting the CI cache means all caches for this project will be deleted, which wastes a lot of CI time on re-populating all the caches that were fine. What I wish I could do it tell opam to simply remove these files anyway, so that CI can proceed without manual intervention. So it would be great if there was some kind of flag that overruled this "files that changed since" check. :)

# opam config report
# opam-version      2.0.8 
# self-upgrade      no
# system            arch=x86_64 os=linux os-distribution=debian os-version=10
# solver            builtin-mccs+glpk
# install-criteria  -removed,-count[version-lag,request],-count[version-lag,changed],-changed
# upgrade-criteria  -removed,-count[version-lag,solution],-new
# jobs              20
# repositories      4 (http), 1 (version-controlled) (default repo at c7b1bb0d)
# pinned            3 (git), 4 (rsync)
# current-switch    ocaml-base-compiler.4.08.1

[dra27]

Are the CI jobs running inside containers or on a file system which doesn't store timestamps properly? You could try setting OPAMPRECISETRACKING to 1 which uses checksums instead of timestamps.

Probably relevant comment in #3997 (comment).

[RalfJung]

They run inside Docker, and we are setting OPAMPRECISETRACKING to 1 already.

# Prepare OPAM configuration.
export OPAMROOT="$(pwd)/_opam"
export OPAMJOBS="$((2*$CPU_CORES))"
export OPAMEDITOR="$(which false)"
export OPAMRETRIES=3
# Work around some OPAM bugs/quirks.
export OPAMDROPWORKINGDIR=1
export OPAMPRECISETRACKING=1

[kit-ty-kate]

#4917 is only a help for this issue, not a fix