fix(.ci-macosx.sh): Avoid { set -e; c1 && c2; } bug & Improve script

* Make it uniform / job(.github/workflows/static-builds.yml : static-bin-macos)

* Add commands to print OS version details

* Don't hardcode `4.12.1` anymore

* Add `opam … -v` flag to ease debugging

* See also https://github.com/coq-community/docker-coq-action#pitfall-do-not-use--use-semicolons

.ci-macosx.sh

@@ -2,15 +2,20 @@
 
 set -e
 
+sw_vers
+system_profiler SPSoftwareDataType
+uname -a
+
 brew update
 brew install pkg-config
 brew install opam
 brew install libev
-opam init -y --compiler=4.12.1
-eval $(opam env)
+opam init -y -a --bare
 
-opam install -y -j 2 . --deps-only --locked
-make && make opaminstall
+opam switch create . ocaml-base-compiler --deps-only --locked -y -j 2 -v
+eval $(opam env)
+make
+make opaminstall
 
 # See src/main/linking_flags.sh
 make detect-libs