Fix opam-* < 2.0.7 when the CI environment variable is set

[kit-ty-kate]

e.g. in opam-repo-ci

#=== ERROR while compiling opam-core.2.0.6 ====================================#
# context              2.2.0~alpha2~dev | linux/x86_64 | ocaml-base-compiler.4.14.1 | file:///home/opam/opam-repository
# path                 ~/.opam/4.14/.opam-switch/build/opam-core.2.0.6
# command              ~/.opam/opam-init/hooks/sandbox.sh build make opam-core.install
# exit-code            2
# env-file             ~/.opam/log/opam-core-7-6bdbb2.env
# output-file          ~/.opam/log/opam-core-7-6bdbb2.out
### output ###
# dune build  -p opam-core opam-core.install
# (cd _build/default && /home/opam/.opam/4.14/bin/ocamlc.opt -w -40 -w +a-4-40-42-44-48 -safe-string -warn-error A-3 -g -bin-annot -I src/core/.opam_core.objs/byte -I /home/opam/.opam/4.14/lib/ocamlgraph -I /home/opam/.opam/4.14/lib/re -I /home/opam/.opam/4.14/lib/seq -I /home/opam/.opam/4.14/lib/stdlib-shims -no-alias-deps -o src/core/.opam_core.objs/byte/opamStubsTypes.cmo -c -impl src/core/opamStubsTypes.ml)
# File "src/core/opamStubsTypes.ml", line 1:
# Error (warning 70 [missing-mli]): Cannot find interface file.
# make: *** [Makefile:104: opam-core.install] Error 1

This behaviour was fixed in 2.0.7 with https://github.com/ocaml/opam/pull/4143/files#diff-90d08e583c4c9c6f391b2ae90f819f600a6326928ea9512c9e0c6d98e9f29ac2R3980