Scripts: make opam env invocation more robust

Tells `opam env` to explicitly use the init script for `sh`, which prevents it
from picking an incompatible script for another shell. In particular, the init
script for `fish` makes no sense when evaluated in `sh`.

scripts/install_build_deps.sh

@@ -22,7 +22,7 @@ if [ ! -d "$src_dir/_opam" ] ; then
     exit 1
 fi
 
-eval $(opam env)
+eval $(opam env --shell=sh)
 
 if [ "$(ocaml -vnum)" != "$ocaml_version" ]; then
     opam install --unlock-base ocaml-base-compiler.$ocaml_version