`export SHELL ?= /usr/bin/env bash` does not work

[tisaac]

The documentation is ambiguous (https://www.gnu.org/software/make/manual/html_node/Choosing-the-Shell.html), but it seems that

export SHELL ?= /usr/bin/env bash

does not count as setting the shell.

I'm about to submit a PR with a command for debugging variables: using GNU Make 4.4.1 on Ubunutu 24.04, I get

$ make aedifix-print-SHELL
SHELL is a recursive variable from file set to [/bin/sh]

This matters because we have scripts where @$(SHELL) is used and bash-specific features (e.g. -o pipefail is only supporter in very recent dash).