small fix for conf-mariadb.2

[raphael-proust]

To attempt to fix the windows error observed in #27862

   #=== ERROR while compiling conf-mariadb.2 =====================================#
  # context     2.3.0 | win32/x86_64 | ocaml.5.3.0 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\conf-mariadb.2
  # command     D:\opamroot\.cygwin\root\bin\sh.exe -exc pkg-config --exists libmariadb || pkg-config --exists mariadb
  # exit-code   2
  # env-file    D:\opamroot\log\conf-mariadb-4432-27ba11.env
  # output-file D:\opamroot\log\conf-mariadb-4432-27ba11.out
  ### output ###
  # + pkg-config --exists libmariadb
  # Can't open perl script "/cygdrive/c/Strawberry/perl/bin/pkg-config": No such file or directory
  # + pkg-config --exists mariadb
  # Can't open perl script "/cygdrive/c/Strawberry/perl/bin/pkg-config": No such file or directory

[raphael-proust]

macos seems to alsoi fail (from the same PR)

#=== ERROR while compiling conf-mariadb.2 =====================================#
# context              2.3.0 | macos/x86_64 | ocaml-base-compiler.4.14.2 | file:///Users/mac1000/opam-repository
# path                 ~/.opam/4.14.2/.opam-switch/build/conf-mariadb.2
# command              ~/.opam/opam-init/hooks/sandbox.sh build sh -exc pkg-config --exists libmariadb || pkg-config --exists mariadb
# exit-code            1
# env-file             ~/.opam/log/conf-mariadb-46834-667a7d.env
# output-file          ~/.opam/log/conf-mariadb-46834-667a7d.out
### output ###
# + pkg-config --exists libmariadb
# + pkg-config --exists mariadb

[mseri]

Still failing on windows, at least in the CI. What should we do?