build: Convert environment into a sequence of bwrap arguments
build: Convert environment into a sequence of bwrap arguments

This means we can systematically pass the environment variables
through bwrap(1), even if it is setuid and thus is filtering out
security-sensitive environment variables. bwrap itself ends up being
run with an empty environment instead.

This fixes a regression when CVE-2021-21261 was fixed: before the
CVE fixes, LD_LIBRARY_PATH would have been passed through like this
and appeared in the `flatpak build` shell, but during the CVE fixes,
the special case that protected LD_LIBRARY_PATH was removed in favour
of the more general flatpak_bwrap_envp_to_args(). That reasoning only
works if we use flatpak_bwrap_envp_to_args(), consistently, everywhere
that we run the potentially-setuid bwrap.

Fixes: 6d1773d2 "run: Convert all environment variables into bwrap arguments"
Resolves: https://github.com/flatpak/flatpak/issues/4080
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=980323
Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak