compilation fails on osx workflows.

[craff]

#=== ERROR while compiling conf-libssl.4 ======================================#
# context     2.1.4 | macos/x86_64 | ocaml-base-compiler.5.0.0~rc1 | git+https://github.com/ocaml/opam-repository.git
# path        ~/work/simple_httpd/simple_httpd/_opam/.opam-switch/build/conf-libssl.4
# command     ~/.opam/opam-init/hooks/sandbox.sh build sh -ex ./homebrew.sh check
# exit-code   1
# env-file    ~/.opam/log/conf-libssl-6769-0daa36.env
# output-file ~/.opam/log/conf-libssl-6769-0daa36.out
### output ###
# ++ brew --prefix openssl
# + brew_pkg_config=/usr/local/opt/openssl@3/lib/pkgconfig
# + case "$1" in
# + test 1 '!=' 1
# + export PKG_CONFIG_PATH=/usr/local/opt/openssl@3/lib/pkgconfig:/Users/runner/work/simple_httpd/simple_httpd/_opam/lib/pkgconfig
# + PKG_CONFIG_PATH=/usr/local/opt/openssl@3/lib/pkgconfig:/Users/runner/work/simple_httpd/simple_httpd/_opam/lib/pkgconfig
# + pkg-config --print-errors --exists openssl
# Package openssl was not found in the pkg-config search path.
# Perhaps you should add the directory containing `openssl.pc'
# to the PKG_CONFIG_PATH environment variable
# No package 'openssl' found