opam should scrub 2.1-specific environment variables in the build

[dra27]

opam 2.1 sets OPAMCLI=2.0 for package builds (#4492). If the user has set an opam 2.1-specific variable in the environment then this leaks to the package build and causes a warning for any calls to opam itself within the package's build instructions.

We should scrub environment variables (with a note to the debugging log) which aren't available in the 2.0 CLI.

The build environment for a package is constructed here:

opam/src/client/opamAction.ml

Lines 483 to 497 in a70ed4f

	let compilation_env t opam =
	let open OpamParserTypes in
	OpamEnv.get_full ~force_path:true t ~updates:([
	"CDPATH", Eq, "", Some "shell env sanitization";
	"MAKEFLAGS", Eq, "", Some "make env sanitization";
	"MAKELEVEL", Eq, "", Some "make env sanitization";
	"OPAM_PACKAGE_NAME", Eq,
	OpamPackage.Name.to_string (OpamFile.OPAM.name opam),
	Some "build environment definition";
	"OPAM_PACKAGE_VERSION", Eq,
	OpamPackage.Version.to_string (OpamFile.OPAM.version opam),
	Some "build environment definition";
	"OPAMCLI", Eq, "2.0", Some "opam CLI version";
	] @
	OpamFile.OPAM.build_env opam)

and here:

opam/src/state/opamEnv.ml

Lines 225 to 236 in a70ed4f

	let updates_common ~set_opamroot ~set_opamswitch root switch =
	let root =
	if set_opamroot then
	[ "OPAMROOT", Eq, OpamFilename.Dir.to_string root,
	Some "Opam root in use" ]
	else []
	in
	let switch =
	if set_opamswitch then
	[ "OPAMSWITCH", Eq, OpamSwitch.to_string switch, None ]
	else [] in
	root @ switch