run: Convert all environment variables into bwrap arguments

This avoids some of them being filtered out by a setuid bwrap. It also
means that if they came from an untrusted source, they cannot be used
to inject arbitrary code into a non-setuid bwrap via mechanisms like
LD_PRELOAD.

Because they get bundled into a memfd or temporary file, they do not
actually appear in argv, ensuring that they remain inaccessible to
processes running under a different uid (which is important if their
values are tokens or other secrets).

Signed-off-by: Simon McVittie <smcv@collabora.com>
Part-of: https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2