openssl invocation: Fix fallback (part of #4448)

[Blaisorblade]

• Please update master_changes.md file with your changes.

[Blaisorblade]

Before this fix, the stacktrace (over 51c160b) showed an uncaught (Permission_denied cmd) exception, Raised at file "src/core/opamSystem.ml", line 44, characters 2-31. Then the fix was easy.

Full trace.

$ OCAMLRUNPARAM=b dune exec --profile=release  --promote-install-files -- opam repo -v -v -v list
'opam repo -v -v -v list' failed.
Fatal error:
"openssl": permission denied.
Backtrace:
  Raised at file "src/core/opamSystem.ml", line 44, characters 2-31
  Called from file "src/core/opamSystem.ml", line 571, characters 4-96
  Called from file "src/core/opamHash.ml", line 99, characters 10-79
  Called from file "src/core/opamHash.ml", line 118, characters 34-49
  Called from file "src/state/opamFileTools.ml", line 1035, characters 20-75
  Called from file "list.ml", line 117, characters 24-34
  Called from file "src/state/opamFileTools.ml", line 1030, characters 10-488
  Called from file "src/state/opamFileTools.ml", line 1067, characters 28-79
  Called from file "src/state/opamFileTools.ml", line 1078, characters 2-15
  Called from file "src/state/opamRepositoryState.ml", line 76, characters 14-68
  Called from file "array.ml", line 158, characters 9-30
  Called from file "array.ml", line 158, characters 9-30
  Called from file "src/state/opamRepositoryState.ml", line 105, characters 14-58
  Called from file "src/state/opamRepositoryState.ml", line 219, characters 12-64
  Called from file "src/core/opamStd.ml", line 292, characters 14-28
  Called from file "map.ml", line 295, characters 20-25
  Called from file "map.ml", line 295, characters 10-18
  Called from file "map.ml", line 295, characters 10-18
  Called from file "src/core/opamStd.ml", line 291, characters 6-72
  Called from file "src/state/opamRepositoryState.ml", line 216, characters 6-416
  Called from file "src/core/opamFilename.ml", line 421, characters 14-45
  Re-raised at file "src/core/opamStd.ml", line 1360, characters 4-38
  Called from file "src/state/opamRepositoryState.ml", line 272, characters 11-23
  Called from file "src/state/opamGlobalState.ml", line 170, characters 14-18
  Re-raised at file "src/core/opamStd.ml", line 1360, characters 4-38
  Called from file "src_ext/cmdliner/src/cmdliner_term.ml", line 27, characters 19-24
  Called from file "src_ext/cmdliner/src/cmdliner.ml", line 27, characters 27-34
  Called from file "src_ext/cmdliner/src/cmdliner.ml", line 118, characters 32-39
  Called from file "src_ext/cmdliner/src/cmdliner.ml", line 148, characters 18-36
  Called from file "src_ext/cmdliner/src/cmdliner.ml", line 266, characters 22-48
  Called from file "src/client/opamCliMain.ml", line 336, characters 8-60
  Called from file "src/client/opamCliMain.ml", line 248, characters 6-10

[rjbou]

Thanks!